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Frequently it comes up that characters will be talking to an NPC and one of them will say something similar to "Do I believe him?" "Roll an insight check."

If they're lying, it's simple: an Insight vs. Bluff check. If they're telling the truth though, what do they roll against? The other person isn't bluffing, so it's not Bluff. As far as I can see, there's nothing in the rules that talks about how to handle a roll to believe an honest person.

What I'm looking for is a mechanic where players who fail an Insight check will think that someone (who is telling the truth) is lying. The goal is to avoid the players (not characters) knowing whether the NPC is telling the truth by presence or absence of a roll, and what to do with the roll if they use insight on someone who's not bluffing.

My first thought is make it a DC 10 insight check by default. Is there a better way?

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  • \$\begingroup\$ Related and possibly a duplicate: How do you deal with metagaming based on die results? \$\endgroup\$ – SevenSidedDie May 28 '15 at 19:28
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    \$\begingroup\$ Do you mean Deception versus Insight contest? Bluff not in 5e rules. Under Charisma checks there is Deception. Your Charisma (Deception) check determines whether you can convincingly hide the truth, either verbally or through your actions. This deception can encompass everything from misleading others through ambiguity to telling outright lies. Typical situations include trying to fast-­‐‑ talk a guard, con a merchant, earn money through gambling, pass yourself off in a disguise, dull someone’s suspicions with false assurances, or maintain a straight face while telling a blatant lie. \$\endgroup\$ – KorvinStarmast Apr 26 '16 at 14:39
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What a PC believes shouldn't be determined by a dice roll.

Instead, give them clues depending on the results of their insight check. For instance, if the PCs are interrogating an NPC regarding a string of robberies, an Insight Check DC10 might allow the PCs to notice that the NPCs eyes widened when shown the torn scarf found at the mayor's house, despite him denying seeing it before (or do an opposed bluff vs insight if the NPC has a very good poker face).

This way, you can play with their perceptions a lot. Maybe the NPC they're talking to knows who this scarf belongs to... or maybe he just realized it was a set-up, as it was stolen from him a few days ago, but would rather deny everything in front of the PCs to take care of the matter himself.

Like in real-life, your players will have to decide what they do with the information available to them. If their Insight check is too low, you might give them only part of the information (the NPC seems to be very nervous, which really could mean anything...), which might in turn lead them to make the wrong decision.

This also seems to be suggested by the PHB (p.178) :

Insight :

Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature, such as when searching out a lie or predicting someone’s next move. Doing so involves gleaning clues from body language, speech habits, and changes in mannerisms.

A good insight check should reveal these clues, but the interpretation should be left to the player.

To specifically address the situation of Insight vs. "Not bluffing", you can interpret the insight check as someone trying really hard to find clues about someone's lies. And when they search too hard, people always end up finding something... So if they roll too low, you could just give them wrong cues about the NPC's behaviour (he seems to be sweating a lot... but that's because he was running to get home before his wife sees her surprise birthday gift, which you don't tell them). As to what could be regarded as "too low", there is a very good guideline in the PHB (p.174) to help you set DCs for such tasks.

Typical difficulty classes

  • 5 --- Very easy
  • 10 --- Easy
  • 15 --- Medium
  • 20 --- Hard
  • 25 --- Very hard
  • 30 --- Nearly impossible

For example, if you think it would normally be very easy to spot someone's lie (Pureferret's example of a paladin whose very nature is against lies), the DC to notice they are speaking truthfully would be 5. On the other hand, if it would normally be hard to spot the lies (the guy from local fence is so used to lying that you can't really tell if he's lying or not anymore, whether you expect him to or not), then the DC should be 20.

As a DM, it is up to you to set up the DCs for such task. Even if the person isn't lying, success could give some behavioral clues to something else ("he looks distracted, as if he's preoccupied with something else") that could be, or not, a plot hook. On the other hand, failure wouldn't reveal any clues whatsoever, and you could rule that a failure by 5 or more (rolling 15 or under against a DC of 20, for example) leads to wrongful interpretation. For example, when dealing with the guy from the local fence, the PCs are expecting him to lie so much that everything seems to indicate it ("he's never looking at you in the eyes, and often checks down the alley behind you" -- of course! he's dealing with illegal goods and is checking for the town watch!).

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Rubiksmoose Jul 14 at 13:03
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You're running into one of the classic hiccups to D&D - it's not primarily a game about intrigue and so the rules for intrigue are...sparse and incomplete.

Here's one solution: Don't roll for truth or lies. Roll to guess motivation.

"He wants this... but there's something he's not telling you. You get the feeling there's a personal drama or angle in this for him."

"Whether or not she's telling you everything, she wants revenge, she wants blood. That part is absolutely clear."

"No, they're not telling you what happened, but you feel this is a debt of honor involved in this, and folks like that will probably uphold their end of the bargain."

So, you're rolling against either their Bluff or a raw D20 if they're being straightforward (perhaps, modified if circumstance makes you unlikely to trust them), and success/failure determines whether you get the real motivations or the motivations you project/suspect/misread from them.

The higher the total, whether it's a success or failure, the more subtle the motivations you are picking up or thinking you're seeing (which also means if they're bluffing, they're damn good at making you think you're seeing through their scam but there's another layer below it.)

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If one of my players where to say "Do I believe him?" I would say "I don't know, that's up to you (or your character)."

If they asked "Does he look like he's lying?" Then I would say "Roll an Insight check."

To do it properly, I would roll an Insight check on their behalf and report the result. If they succeeded, I would say "You detect this or that sign that makes you think he is telling the truth / lying" (depending on which he is doing). If they failed, I would say "You find him hard to read" or similar.

If they had a really strong success, I would give more useful detail. If they had a really strong failure, I would give them misinformation: for example, if he was lying I might say "As far as you can tell, he is telling the truth... he strikes you as a very honest person".

The point is, "Do I believe him?" Is not a fair question for players to ask. The proper procedure is for you to describe the scene, then for them to ask questions which provide them with further detail as their characters look closer at those aspects of the scene in which they are interested. It's not your job to tell them how they feel about what they see; that's up to them to roleplay.

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... when the other person is telling the truth?

5e has three social skills: Deception, Intimidation and Persuasion. To me, the Deception skill includes convincing someone that you are telling the truth. Convincing someone that something is true may be no more or less difficult than convincing them that something is false, regardless of reality.

Consider if the band of PCs comes running into the city with dire warnings that the nearby Dragon is coming to burn down their homes. The important thing is not necessarily that the PCs are telling the truth, but that the NPCs believe the PCs. (For all the NPCs know, these people could just be looters trying to scare them from their homes.)

Normally, with NPC interrogation, I let the PCs do some of the roleplaying for this stuff. If the PCs are really insistent that the NPC is lying / truth-telling, I'm not going to override that conviction. They can play that out.

However, PCs (and NPCs) also have skills that may go beyond both the player and the DM. I may be a terrible actor and the PC may be an excellent Investigator, so when the player is not certain (or grievously wrong) we let the dice help us along. I'll roll a Persuasion vs Perception/Investigation.

Depending on the result of the check, some or all of the PCs will be convinced/unconvinced and we play it out from there. If the party is split on opinions, they make take a different tack (Intimidation, new questions, ...) or someone will relent.

On the downside, this may cause dice to affect what the PCs believe. But then, a particularly dim Paladin may really be deceived by a charismatic Rogue. Sometimes a PC's weakness does have to cause them some troubles.

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    \$\begingroup\$ Why deception and not persuasion? \$\endgroup\$ – mattdm Jul 14 at 17:28
  • \$\begingroup\$ Deception is about making somebody believe what you say. It doesn't really matter if the thing you say is true of false, though that may modify the roll, it only matters that they believe the thing you told them. Persuasion is about convincing people that you're a great person, that you're their friend. You can do this without trying to convince anyone that something is "true or false". Now, the two can sometimes inform each other. Like my friend might go along with someone I say even though they know I'm lying. But people will react differently in both circumstances. \$\endgroup\$ – Gates VP Jul 17 at 15:34
  • \$\begingroup\$ That doesn't square with what the rules say. Specifically, deception is described very clearly as being about hiding the truth. "Your Charisma (Deception) check determines whether you can convincingly hide the truth, either verbally or through your actions. This deception can encompass everything from misleading others through ambiguity to telling outright lies.", while "Typically, you use persuasion when acting in good faith, to foster friendships, make cordial requests, or exhibit proper etiquette." \$\endgroup\$ – mattdm Jul 17 at 16:04

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