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Most of the fantasy systems I know has a spell to detect lies, speak with the dead, and the like.

Imagine how long great movies like The Usual Suspects or In the Heat of the Night would have lasted if the detectives had access to Detect Lies or Analyze Truth.

How can you deal with this problem? I met several wildly different solutions, from outright banning similar spells, to limiting the spell's scope to yes/no questions, to avoiding whodunit storilines alltogether, and focusing on Why done it.

None of these seem to be the perfect solution to me, how do you handle this?

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This seems to be related to questions such as "How can you deal with magic unbalancing the political/economic situation". Perhaps related questions can give you some ideas on how to deal with it. –  lisardggY Feb 27 at 9:26
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As a sidenote: there is always elements breaking stories. The entire Magnum, PI detective series could be reduced by about 90% if they'd had cellphones at that time. Cellphones would have broken almost all stories. And yet there is a very good Sherlock Holmes BBC series that's set in todays world and works even with computers and cellphones. I guess it's a matter of story selection. –  nvoigt Feb 27 at 9:46
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The writers of "Lie to Me" had the same problem. Main character can always tell when people are lying. How then to have two seasons of him? What it comes down to, is that he doesn't know why they're lying, or which part is the lie. Several times he's questioning a suspect, detects they're lying like crazy, and only later learns they're trying to cover up an affair and are indeed completely innocent of the crime. –  Mooing Duck Feb 27 at 17:09
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Not long enough to be an answer: You cast Detect Lies on Bob and tell me he's lying. Why should I trust you? :) –  Allen Gould Feb 27 at 20:15
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Confront the PCs with characters that only have incomplete / inaccurate / need-to-know information. Think guerilla cells. "We get our orders from a dead drop", where the dead drop will obviously not be used if the party is stalking it and not a known contact. Another way of looking at this is: you work off a whitelist of clues the PCs are meant to know at a given stage of the adventure. So instead of designing NPCs that know "everything" and deny information to PCs, design NPCs that only know what the PCs need to progress. (That said, the danger here is that NPCs will feel contrived.) –  millimoose Mar 1 at 18:52

12 Answers 12

Supernatural poker face

The first option seems to be: give immunities to important liars. But there are two problems on it:

a) Being immune to Detect Lies makes you automatically suspect of being a liar, just as someone with his face covered seems to be a criminal.

b) As more and more of your NPCs have that immunity, it won't be credible, especially if it comes from people without strong magic/supernatural background.

Increase the cost

This can't be done on all systems, but if you can it's not a bad choice. You don't forbid the use of the power, but you make it costly enough that players have to think twice when using it. You won't avoid them checking obviously important people, but you will avoid them scanning every single sentence a NPC says.

Make the use uncomfortable, or obvious

Imagine that you can magically check if people around you are lying. You would use it, right? Imagine that for checking them, you have to sit them on a chair and attach cables to them to monitor their vital signs. Would you still use it?

The spell could require a ritual as uncomfortable as that process. That would make it not suitable to all situations. But it will still be useful for others.

Alternatively, it may not be so uncomfortable, but rather obvious. Maybe the mage has to make magical signs with the hands, or look the suspect through a glass. Or the victim can feel the mind of the mage scanning him. Anyway, this will make the use of the spell rude on most social situations, as it would be akin to an interrogation.

If the characters don't have the right to perform such interrogations, the act can even be considered illegal. There's also a risk on warning the victim that he is under suspicion. An obvious detect lies spell is a quick way to make enemies.

Example: The Bone of Truth rite on Vampire: The Masquerade. Suspect must hold a bone, which will turn black if a lie is said. It is useful, but only on interrogations. You can't make an Elder hold the bone while you have a casual conversation with him.

Blackbox

Don't make the spell too accurate. Let the suspect talk, then inform the player things like: "he is hiding something" or "not all he said is true". That way, players will know someone has lied to them, but they don't know what is true and what is false, and anyway few people are 100% sincere.

If players want to be more specific, you can ask them to make several spell use, paying each time the costs. You can also improve the quality of the information with the success degree.

Train the players not to trust too much in the information

Related with the previous one, there are lies that hides truth and truths that hide lies. In "The Name of the Rose" inquisitorial interrogation, a man says a lot of truths on this fashion. I don't want to spoil the book, so I will make my owns:

  • I swear I didn't kill my wife. (True! He did kill his wife, but it's true that he swear he didn't).

  • I didn't steal the crown. (True! He paid someone else to steal it, then bought it).

  • I don't have the crown. (It's buried on some place I only know).

In the same way, there are truths that can be detected as false because of small lies.

Also, if someone has been fooled, or he is simply wrong about something, what he says is not true, but it isn't a lie, because he thinks it's true. If your players give it 100% credibility just because the spell, they will make a huge mistake. Better still, if the players detect a lie that is in reality a truth, they will be highly disoriented.

Design your adventures counting on the spell

At the end of the day, if you don't forbid the spell you must face the situation that your players will detect some of the lies.

Make it useful, but don't let it ruin your adventure. Count that the players will be able to detect lies, so don't give any lie the key of your adventure success.

The mastermind, the one who knows all the truth, can be someone hidden, operating on shadows. His servants won't know all the truth. Still better, they have been lied in several ways. Any interrogation will lead to true and false information, so the players will have to investigate which is which.

There are a lot of people that have been fooled by that villain (starting by his own henchmen, but the list can include some honest, respectable and good willing citizen), so their information is corrupted.

Some important communications can be written. Usually detect lies don't work on a letter.

Final advice

If you allow the spell, don't make it completely useless. You can restrict it a lot, as seen above, but make it still useful or your players will feel scammed. Warn your players the spell have limitations (although don't explain everyone) before they get it.

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Also, what of lying for a lack of knowledge ? "She is not my sister". -False, but Luke didn't know... –  Nigralbus Feb 27 at 10:36
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In addition to uncomfortable and obvious, you might add "socially unacceptable" or even down-right illegal in some cultures, perhaps lumped in with mind-reading and charms. –  Zimul8r Feb 27 at 13:35
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+1 for "truths that hide lies." True statements don't always have to convey true facts in an obvious manner! Misdirect the player; after all, any shady character in a setting with "Detect Lies" would learn to avoid lying... but they still need to avoid saying what they mean! Silence or something like reiterating Name-Rank-Country and nothing else is also an option for someone unwilling to give up secrets. –  Brian S Feb 27 at 15:00
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A lie for the lack of knowledge should detect as a truth. Assuming the spell is designed to detect lies, it can only tell if the speaker is lying. If Like doesn't know that Leia is is sister, then he is not really telling a lie in denying it. Think of it this way--the ability is not verifying the factual accuracy of the statement, but rather the intent of the teller. –  otto Feb 27 at 21:52
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@otto: Without your interpretation, the PCs could cast Detect Lies on each other as a cheap way to ask questions about the whole adventure. "Where is the treasure hidden?", "It's in a chest.", "Correct. Where is the chest?", "It's in a dungeon.", "That's a lie", "Err...ok, it's in a castle . . .", "Correct . . ." :-S –  Neil Slater Feb 28 at 9:03

Lie Detection Barrier and Misinformation

If there is a way to detect lies, then there should be a way to protect against lie detection. Villains would protect themselves, and would not share the truth with their minions. In this case they may even lie to his/her high-rank generals, because if someone try to detect lies, they would find none. They may even give a false version of something and then a yet another false version, to function as a cover story. The heroes would detect a lie, but as soon as they find what the general believe to be true, they would face yet another lie -- this time, undetectable.

"Try whatever you want, I won't say a thing"

Also, any antagonist of your story could do the same as people usually do in the 24 thriller: just reffuse to say anything. In 24, people do not have access to magical lie detection, but they use scientifical/technological lie detection all the time. Usually, the villains in 24 have a greater cause, and will face even torture in order to protect a lie. Villaisn have even said, during torture, that they prefer to die as martyrs for their cause.

Hide behind the masses

Usually, even if able to detect lies, spellcasters can't do that all the time. There will be a practical limit of detections per day, so if they can't find who may know the truth, then any detection spell will be useless.

If everything fails, then proceed to the backup plan

Villains also should not trust that their servants will be able, or even willing, to keep their share of secrets. Always have a backup plan. Maybe on of the gurads or soldiers work for him, and they should kill anyone who gets caught and may be forced to talk too much. Or even if the heroes find the truth, use this as an oportunity to an ambush, when they go check it out.

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Change story focus.

Just catching somebody on lies is not automatically win. It's a clue. From standpoint of Unusal Suspects: investigators know that this person lies to them. They just don't know what part of this lie is correct. And they need to know curcumstances to charge somebody with something.

Detect Lies is a glorified error prone polygraph (lie-detector). I will solve marginally small crimes (Did you steal this spoon?) or can get you magical conviction from murderer (Did you kill this man?). But it will not solve complex investigation by itself. You will need to do bunch of finding and apprehending supects before Detect Lies will even be applyed to them. And they can always die before somebody can lie detect them.

Also as a sidenote. For real magical provechecking you need two people with Detect Lies mechanism. First one checking the suspect and second one checking the checker truthfulness. This can be very funny chain of checks to establish that someone is truly lying with aid of magic.

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Along with the "error prone polygraph" approach, I think that dice roll checks like this are things that are best made in secret by the GM. You can try to detect lies and get a result, but you won't know if you rolled a 2 or a 20. If you know the roll, then you are probably going to be tempted to metagame based on how successful you think your check was. –  otto Feb 27 at 21:57
    
The "two people" method doesn't make sense: now you've just moved the burden to someone else, doesn't really solve anything. –  Lohoris Apr 19 at 10:06
    
@Lohoris two people method is basically legal separation procedure for magical society with judicial principles. And you absolutely right i doesn`t solve anything really and just move burden to some one else. Real polygraph work this way. There is always expert to say what this graphs really means. –  aardvark Apr 20 at 14:53

Frank Herbert's novel Dune touched on this. There were people who could detect lies unerringly (the Bene Gesserit "Truthsayers"); as a result, the unsavory powerful people arranged their affairs such that they would not have to lie. They wouldn't say "Execute this guy and get rid of the body" to their underlings; instead they would say something like "I never want to see this guy again" and the underlings knew what to do. Consider the Q & A:

Did you kill the guy? No. Did you order his death? No. Do you know where he is? No. Do you know whether he is alive or dead? No.

The unsavory person is at least 99% certain that an underling executed the guy and got rid of the body, but doesn't know this and the above answers are all truthful.

(This does raise a point: IMHO there should be standard questions like "Where was the guy the last time you saw him?" and "What were the last words you heard him say?" Or even "Do you have any reason to think he might be dead?" The Truthsayers should up their game in a world like this.)

There is a related question: psionic abilities and spells like "Clairvoyance" would allow solving many mysteries. "Where's the McGuffin?" someone casts Clairvoyance "Oh, there it is."

This was touched on by Randall Garrett in one of the Lord Darcy stories. Since it would be intolerable to have one's privacy constantly invaded, people invest in privacy shielding spells, and any city has so many privacy spells packed close together that magicians can't just use their powers to instantly find out who committed a crime. (If a murder were committed out in the middle of a field way out in the country, any Journeyman magician can just see what happened and say who did it "as easily as he would find a strayed sheep.")

It seems likely, to the point of certainty, that rich and powerful people in a fantasy milieu would have countermeasures to the truth-detection spells. Possibly there should be a spell, "Cloak the Truth", that would make someone immune to truth-detection. And the powerful people would have that cast every day.

Then in turn, there would be a judicial procedure where someone in a trial might have the official court mage dispell any cloak spells before testimony... but in turn, there might be a set of laws covering when such a procedure could be used, or strictly limiting the questions that could be asked.

So, if the PCs use "Detect Lie" on some random bartender, it would likely work. If they use the same spell on rich and powerful people, it would likely fail. Their use of the spell might be detected and there might be repercussions from that... from as simple as "you are no longer welcome to come here" to as extreme as thugs sent to kill the PCs.

I think there are two extremes where crimes would be difficult. A really commonplace crime, where a guy gets attacked from behind in the worst part of town, would have no real leads. (You could get an honest answer with the spell, but on whom should you cast it?) And any crime involving the powerful, who might have countermeasures, would be difficult due to the countermeasures.

In fact, a "Detect Lie" spell can expedite the plot a bit. A woman is killed, and everyone suspects the estranged husband, but "Detect Lie" says he is telling the truth that he didn't kill her and is sad that she is dead. Now the PCs can ignore him and get on with the investigation the game master wants to occur.

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+1 - Excellent answer! –  wraith808 Mar 1 at 2:57
    
I'm not sure why it got so few votes compared to the others. –  Lohoris Apr 19 at 10:11

You group them all together but I handle these things three different ways:

  1. For effects like detect lie, my rule of thumb is that it only works if the statement is a literal falsehood, and only reports to the user that a lie is being told - not what part of the statement is a lie or what the truth is. Intentional deception that is technically true doesn't count, so "I have no intention of harming you when you get back from the mission" wouldn't be detected if the speaker is paying someone else to do the harming, or (more deviously) if the speaker has already done something to harm the user that will only take effect or be realized once the user comes back. Rhetorical questions enable you to get away as well. The PCs can always badger their targets to state things more clearly, but it's not very diplomatic and can get them in trouble.
  2. For effects like speak with dead, I generally assume the entity retains its own personality and prejudices, and therefore may or may not wish to mislead whoever is referring to it. It won't have any knowledge of the future, since the point of the game is to decide the future through gameplay, and it won't know anything it would be unreasonable for it to know in the first place. A long-dead king could tell you where his magic crown was hidden, for example, but not where it is now. He might or might not be able to say if a particular individual was also still alive, but that depends on whether their spirits had met in whatever afterlife or plane (i.e. it would be a percentage based on alignment and religion). If the spell is being abused, I can have the power that be answer in riddles or vague statements instead of straight up, but if it's something players have shown more reluctance to take advantage of I generally reward their discretion with better information.
  3. For effects that are specifically meant to predict the future, I use a special mechanic. Instead of getting direct answers when characters do that, either by themselves by talking to a fortune-teller/soothsayer/seer/etc, they get a "fortune card", which is an index card with some kind of traditional folk-divination element - I Ching hexgram, Tarot card, zodiac sign, etc - along with traditional information about how to interpret it. So you might have The Hanged Man or Difficulty at the Beginning. The player keeps that card - one per person at a time - until one of three things happens:

    • Before a skill roll, the player can explain how this action fulfills the prediction, and if it's reasonable, they get a significant bonus to the roll (normally +d10 on a d20).
    • After a perfect skill roll (e.g. natural 20 on a d20) the player likewise explains how this success fulfills the prophecy, and if accepted they get a bonus on the effect of the roll. Extra damage, extra effects, longer duration, etc.
    • On a natural 1, I as GM get to say how the situation fulfills the prophecy and impose a major penalty (in lieu of the usual botch effect, if applicable).

    In any case, after its effects are work the prophecy is returned to my stash. This gives players an incentive to seek out prophecy and prognostication - which was a major part of life in most pre-modern cultures - that is also balanced out by a potential downside. At the same time, it avoids getting into hairy questions of predestination and free will.

EDIT: I just realized also that sometimes intentionally lying might be a way to avoid being caught by detect lie, if sends the PCs on a wild goose chase. "I did not order my men-at-arms to kill this person" is still a lie if the order was given so quietly no one could hear it or in a language none of the men could understand; the PCs spend all their time trying to prove it was one of the henchmen when the speaker simply did it himself.

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The usual DMing counter against speak with dead in my games was having assassins cut the tongues of their victims. –  Zachiel Feb 27 at 15:34
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Assassins, heck. Cremating corpses might get to be pretty standard practice to prevent their desecration by necromancers. It's always interesting to think about how having all kinds of spells like these floating around must change society. –  Robotman Mar 1 at 19:13
    
@Robotman and it's sad to notice how many games actually don't care and pretend it's not so :/ –  Lohoris Apr 20 at 10:36

Play to the spell's weaknesses. In Robert Jordan's The Wheel of Time, an organization of spellcasters known as the Aes Sedai have mystically bound themselves to an oath to, among other things, "speak no word that is not true". But they have become masters of rhetorical techniques like misdirection, dodging the question, concealing information, and so on, such that the people say "an Aes Sedai never lies, but the truth she says isn't always the truth you think you hear." The oaths were intended to make them trustworthy, yet nobody trusts them.

Most spells like detect lies have limitations that recall this general theme: they only detect deliberate falsehoods. Rhetorical tricks slip through, as long as the user doesn't actually say anything false. Honest ignorance also slips through: "I don't know" isn't a lie if you really don't know. Same goes for sincere mistakes; if you believe something is true when it isn't, then the spell won't flag it as a lie.

Spells can conflict, with interesting consequences. In Lois McMaster Bujold's Miles Vorkosigan series, there exist a drug called fast-penta which prevents people from lying or from holding back information. But some people are allergic to this drug, and if administered, it will simply kill them. These people are highly valued as spies and people who work with classified information, and some people undergo a medical procedure to become allergic to fast-penta so that they can enter this field.

In D&D, you could simulate this by requiring secret agents to accept a geas to lie about their employers or other relevant information. Acting against the geas causes great pain -more to the point, it can even cause death- and so if interrogators try to use magical compulsions to tell the truth, the geas can prevent this by killing the agent. Interrogators thus learn that they're dealing with someone who has powerful magical resources, but they probably already knew that, and as an added tweak to their consciences, they cast the spell that touched off their captive's death.

Make them prove it. The Ace Attorney series of video games presents you with a set of murder stories and mysteries. Rather than playing a detective, however, you play the defense attorney of someone who has been wrongly accused. In some of these stories, you (the player, not the characters) are even shown the murder itself, right at the beginning, including the real killer. But this isn't enough to win: you have to clear your client's name by proving who the real killer is.

The limitations of detect lies should be well-known to D&D legal systems. The statements of a witness under this spell may be admissible as evidence, but because of the ways that someone might fool the spell, deliberately or accidentally, their results don't necessarily constitute hard proof. This turns the story from a "whodunit" into a "howcatchem", but it also means that the results of detect lies is just another lead, not the final nail in the coffin.

Work from multiple conflicting perspectives. In Ryunosuke Akutagawa's In a Grove (and Akira Kurosawa's Rashomon, which is based on it), we are presented with several different views of a murder, including one from the victim himself (as told through a medium). While they all agree on some critical aspects, they conflict to the point of irreconcilability in others. They cannot possibly all be true, and it turns out that everyone involved, even the victim, has reasons to lie. You never find out the true story: Kurosawa's version adds an eyewitness account, which is just as contradictory as the others, and then it turns out that even he has his own reasons to lie, so you cannot fully count on his account either.

That said, these literary scenarios involve a case where people can lie. The divination-proof version of this ties into the limitation-based method above: people think they saw different things, and they relate these honestly, so detect lies never trips. But they can't all be the actual truth, and so now the characters have to sort out what really happened.

Make the truth too far-fetched to believe. This one is great for cosmic horror stories. The truth blatantly violates everything the questioners believe about the world: it cannot be true, yet here the witness is saying it, and you know they're not lying. How can this even be? The witness probably doesn't know, and so now the PCs have a new mystery to solve.

Make the truth too horrible to believe. This one requires a DM and players who are comfortable with mature themes, because now you've got to delve into the nasty stuff. The witness relates a story so horrible, and involving such beloved figures, that no one wants to believe it, possibly not even the PCs. Wars have started, and kingdoms have fallen, over things like this, which puts the PCs into a major ethical dilemma: the truth will have Consequences if it is revealed to others. The example that comes most strongly to my mind here isn't from literature, but from real life: the story of Jerry Sandusky and Joe Paterno. Like I said, mature themes.

Show the PCs just how wrong they really are. As the PCs start to gather information, make sure the perpetrator leaves behind a bunch of false trails and leads, and note which ones the PCs take up. When they go to question the witness, they get a bunch of boring information they though they already knew... until detect lies starts to trip. This is a great way to make the conflict a personal one, because now, not only has the BBEG committed the crime, he has also tricked the PCs themselves. The witness might know more, or you might conveniently "disappear" him, and now the PCs have to figure out what to do in a scenario where they have already been tricked once.

The bottom line is that there are lots of ways to handle lie detection. Even if you can't hide things well enough to make finding the truth a major plot point, you can still construct things such that dealing with the truth is much worse than finding it was.

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Those adventures are perfectly viable for low level characters, that don't have access to game-breaking magic or software/cyberware.

Especially for D&D, but other systems also, characters become too powerful to actually tell believable stories inside the system later on. Personally, I just change those spells. They still work the same way but only give the caster a vague feeling. In addition, people are not inclined to believe a magicians vague feeling. So the caster may know who did it. But that's not enough. He has to prove it. Otherwise, he's just another lunatic accusing random bystanders.

There are a other solutions as well. E6 tries to solve this problem for D&D in their own way, by not granting access to those powerful magic.

There are systems that focus on combat and the power of characters a lot. Those tend to have powerful, game-breaking or at least adventure-breaking magic.

But there are other systems that focus more on social interaction, intrigue and cleverness. In those systems, combat tends to be simpler and more deadly, because it's not the game part of the game, but rather the failed the game part of it.

Example:

Shadowrun favors combat and has magic that is capable of breaking good stories. Cyberpunk favors interactions, where combat is deadly. It's hard to find something story-breaking.

D&D favors combat and has magic that is capable of breaking good stories. Vampire & Co favor interactions, where combat is deadly. It's hard to find something story-breaking.

So there are systems that fit well with whodunnit adventures. Or rather adventures. And then there are systems that fit well with combats lined up with some story in between.

Both are fun, you only have to pick the one that suits your adventure.

Anecdote: I once thought about an adventure where a sunken ship should be salvaged. I found out that there is a mid-level spell in D&D that lifts a sunken ship to the surface and keeps it there. Needsless to say that I did not prepare this adventure for D&D.

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What a great thread!

Having enjoyed it so thoroughly, I think there's one trick that wasn't explicitly mentioned: illusions and false memories.

  1. For example, if the bad guy murders the mayor while creating the illusion that he was the mayor's wife, witnesses afflicted by the illusion would honestly believe that the wife committed the murder. You could be as simple as this classic detective novel disguise example, or much more elaborate. The key is that the witness' memories are accurate but what they think they saw is not what they saw.

  2. More sophisticated would be magic that implants false memories or false beliefs.

In case #1, magically talented/gifted viewers might see through the illusion, or be able to be "hypnotized" to revisit their memories and see a strange inconsistency or effect that gives the clue that it was an illusion.

In case #2 false memories, you might be able to, at great expense or by luck, detect that the memory has been implanted -- perhaps leaving residual magic -- or you might use the old-school method of finding that they are inconsistent. In addition to false memories, a false belief could be implanted and it would be unsupported by actual memories -- though the mind might fill in details with other (real) memories.

I would also guess that case #2 would require high level magic, and case #1 would also require high level magic if there were multiple witnesses or if the illusion had to span a large space or time.

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Obvious solution is obvious

Ban these spells, or make it so there's some way to ignore their effect. In D&D, the glibness spell gave people immunity to zone of truth and a massive bonus to bluff.

Unfortunately, Even with this option things could go bad if not plannned properly. If I were to interrogate a D&D guy I'd make sure he has no magic auras on himself and I'd cast something harmless on him just to be sure he's not someway hiding the presence of auras.

That leads me straight into the second option.

"This is preposterous"

When someone with strong interests in telling a lie and getting away with it, he says it's offensive for him and his family/guild to even be suspected. This only works if the liar is affiliated to some powerful and respectable circle and still does not protect him from other forms of divination that does not rely on the presence of the subject to inquire its honesty.

But if you combine some form of magical protection against lie recognition / divination and you refuse to be analyzed for the presence of these protections, you are pretty safe.

I had an halfling character in my D&D campaign who used a version of this: "you always doubt of halflings, I'm offended by your racism and I won't let you judge me". (And man was it horrible: he was stealing money from the rest of the party...)

Conclusion

If divination spells (like ones who let you ask "who was it who betrayed us?") do exist, it's extremely hard to build a campaign on lies. If one wants to live this sort of world, he does not lie on important things. Truth recognizing spells are not a problem as long as whoever lies can avoid them, possibly at a price.

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Another way, which I used in a game some years ago, depended on the fact that you actually have to ask the suspect a question before you can determine whether or not his answer is or is not actually true. You lose the problem of information you receive possibly not being accurate, but that doesn't obviate the problem of needing to receive information in the first place, or needing to receive the right information, or needing to put said information together. You've still got to work out what questions to ask, in other words.

In fact, the detective character in that same game could also pull information directly from someone's head via magic, eliminating the problem of asking the right questions- but even then there's still a lack of information. Each witness only saw a bit of the actual circumstances surrounding the crime, and none of them had actually seen the murderer.

Of course, once they found a suspect (by noticing that the victim owed a certain gentleman rather a lot of money before he died), confirming that he did the deed was simple- or rather, would have been simple, had the murderer not had a few tricks up his own sleeve, such as invisibility. Can't ask him if he did it if ya can't actually find him.

In short, a story that works in one world with certain rules may not work in another world with different rules, at least not with some modification. The plot to Magnum PI wouldn't work in a setting that has cell phones because it was designed for a setting that did not. So ignore the problems that are solved by the presence of easy communication and add some new ones- perhaps even some that are caused by that same convenience.

Remember that solutions to a problem can often cause problems of their own: magic solves the problem of verifying information, but it also introduces the problem of your murderer being able to strangle his victim without actually being in the same room.

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There are many excellent suggestions here. I would like to expand on one of them, though.

Some systems/settings have spells or abilities which can directly counter Detect Lies (or the equivalent). I recommend, if you end up using this in your campaign, making such counter-tactics obvious to the players, so they know exactly what's going on. Telling the mage "sorry, that doesn't work" is no fun for anyone.

As a specific example, Corrupted Words is a Terrestrial Circle Sorcery in Exalted. The caster chooses a subject while casting the spell, and until the spell is broken, whenever the target of the spell (which could potentially be the caster himself!) even attempts to communicate about that subject -- whether voluntarily, under duress of mundane interrogation, or under the effects of magic -- they begin vomiting up maggots, and become incapable of communicating what they attempted to convey.

This does several things:

  1. It is immediately obvious that the person is under some kind of geas to prevent them from talking. Even if the characters (or players) are unfamiliar with Corrupted Words, the fact that the person they're interrogating starts vomiting maggots when they capitulate should be a clue.
  2. If the character recognize the effects, they know the "other guys" have at least one sorcerer on their side. This gives them some information despite the failed interrogation.
  3. If the party contains a sorcerer, that character ought to know one of Emerald, Sapphire, or Adamantine Coutnermagic, any of which would remove the effects of Corrupted Words and allow the interrogation to continue. (Alternatively, a higher-level necromancer in the party might know Onyx or Obsidian Countermagic.)
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As a side note, I was one a player in an Alchemicals game where we encountered an NPC under the effects of the Alchemical equivalent of Corrupted Words. The NPC was a real jerk, and kept triggering the effect on purpose just to vomit black sludge onto the players. –  Brian S Feb 27 at 15:21

If this is a constant problem for your campaigns, you could always take a nuclear option to mitigate it.

Make it non-permissible by law

Magic after all can be tampered with, and a savvy thief could argue that they were forced to state things they 'believed to be true' by a magical cohort beforehand. This is not to mention the moralistic consequences of prying open a person's mind in this way, or general magical distrust that many people in your magic-included society might have.

True, if your party isn't with the law, they might not be inconvenienced by it that much, but if they DO have to report to a legal authority, they're going to need more than just a "Detect Lies" to get a confession out of them.

And, there's no reason a person who IS under the Detect Lies spell has to say a single word, and you can always make them exercise their right to remain silent under such circumstances.

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