How do you discourage “player knowledge” as a GM?

Question

• As a trivial random encounter is winding down, a player thinks "That was too easy... The GM wouldn't give us an encounter that doesn't give XP," and starts searching for hidden bad guys or other clues.

• There are four statues in a room, and you use miniatures to represent them. The players think to themselves "Well if the GM used miniatures, these things are definitely coming to life," and get ready to fight.

• When you lay down a map of a seemingly random part of the forest trail, the players think "This must be important, let's search around / prepare for battle."

• You roll a 1 for a player's knowledge check and feed them inaccurate information. They saw the roll and know you're full of it.

Even the most disciplined players sometimes will take advantage of "Player Knowledge" over "Character knowledge." Sometimes it's hard to even tell the difference. You know that you should switch to your frost battleaxe against that Fire Demon, but does your city-raised fighter with the intelligence of an old boot know?

What strategies are there to discourage the use of player knowledge, and how can you cut back on perceptions that players have of the genre itself?

Answer 1

From the DM's perspective, it mostly comes down to obfuscation and training your player base. Don't force your players to resist metagaming if you can reasonably help it.

Going down your list with some concrete examples:

• Make truly trivial random encounters a regular (but not necessarily frequent) occurrence. If every encounter is non-trivial, then a trivial one will scream "ambush."

• Again, if you use minis to represent statuary on a regular basis, it will stick out much less.

• Don't bring the map out until it's actually needed (combat starts, the players need more detail to search). If for some reason you need the map before the PCs realize what's going on, find some pretext for using the map tangential to your actual purpose. In addition, if the PCs start doing more than passive information-gathering, ask them why their characters are suddenly acting the way they are.

• Don't use public rolls for knowledge checks if misinformation is a possibility. Realistically roleplaying belief in something you know to be untrue is extremely difficult. Alternatively, instead of giving misinformation, give accurate but incomplete information. In this way the players can rely on what they know, but are left to speculate on what's been left out.

Obfuscation and training will slowly help train your players to metagame a bit less. You can help the process along a bit by explaining things... Talk about how you want to add trivial encounters to make them feel powerful, or minis to help make the map feel more three dimensional.

The trick with talking though, is to be honest. If you talk up trivial encounters and then slam them with an ambush, the explanation becomes part of the metagame.

Answer 2

Embrace meta-gaming! It will always be there, and it's really just a matter of perspective on the shared fiction of your gaming world. The executive summary for my post: let the players have their way. You don't lose much from it, and you can spend your time challenging them in other ways.

The Too Easy Encounter

So your players roleplay their characters as paranoid, twitchy dudes. If the issue is that they are wasting your gaming time, just tell them: you spend an hour searching for enemies and find nothing. There's no rule that says you need to roll the dice if there's nothing at stake.

If the issue is that it's breaking your verisimilitude, give them consequences. NPCs who treat them like paranoids. Make searching waste time and resources. And stop giving them perfectly balanced encounters--give them encounters with verisimilitude, so they can make decisions based on the game world: weak monsters in safe places, strong monsters in dangerous places.

The Statues

Embrace this one. Statues usually come to life in D&D, as do skeletons and most bodies in sarcophagi. Unguarded gems are usually a trap. Harmless damsels, farmers and kindly wizards are usually shapechangers, cannibals and cultists. These tropes are part of the fun of the game. Mess with them! Also, don't make the statues always come to life. They'll be really surprised when the bad guy comes out of the wall opposite the statues and eats the party Wizard, huh?

Laying Down the Map

So the issue here is that your players want to put themselves in the best tactical position, and you want to be fair about that, but you also want to leave open the possibility that they might screw up and put themselves over the lava pit trap.

My solution is, don't put the map down until combat starts, put down any monsters they can see, and then let the players place themselves anywhere reasonable based on the narration until that point. You don't lose much by letting the party demonstrate its superior tactical ability--they are the heroes, after all--and since they might not know where all the dangers are at that point, once in a while you'll get to have your fun when the wizard puts himself right next to the man-eating tree or the doppelganger that's been pretending to be a muleskinner for the last four sessions.

Rolling a 1 on a Knowledge check.

For a knowledge check, this is no big deal. This is the "I should know this" moment. You studied this, but for some reason you're confused as to whether "Gabazzon" is the false passphrase the Cult of Ecstatic Writhing uses to separate novitiates from initiates, or if it's the correct passphrase.

For anything else, it's the same as the "Too Easy Encounter". Let your players be persistent, rude, dismissive, paranoid, etc. Just make it matter! Their snitches get annoyed when their gossip isn't believed, sages are insulted when they are called incorrect, their characters spend hours poking around looking for the trap they're sure is there... it's all good.

Unless the roll didn't matter. Then just tell them they act all paranoid or whatever for nine hours and still find nothing. Or better yet, don't roll.

Embrace meta-gaming. Meta-gaming is to D&D as bluffing is to poker. It's part of the fun.

Answer 3

Consistency, realism in the rest of the game, dice rolling privately where necessary, and reinforcing the roleplay aspect are the 4 major pieces of advice I would give.

• Consistency is needed in the interchange of information between GM and Player. It is not the only thing, but it helps to be as consistent as you can. Use the map regularly in the middle of the forest just to 'see how the marching order looks' as a matter of course, and they will think nothing of it. Use miniatures to represent inanimate things more often, or just mark them in with marker before they animate...when you fool the players more often, they stop assuming they know what is coming. This also means consistently use differing levels of random encounters.

• Realism in the rest of the game is probably one of your best subconscious weapons. If the players know that it is more important and rewarding to focus on in-game knowledge, they will start cuing on it more readily. If it is an orc infested forest, and the group runs into lots of orc groups, they won't worry about the exp or other metagaming thoughts. Similarly with the statues, if they know they are in the lair of a golem maker, they'll use in-game knowledge to work out the statues or not. Even your lousy knowledge check can be used if you feed one player information that another player already knows cannot be true. If you have done a good job on creating a regular knowledge base, it is easy to add an obvious, in-game logic explanation for them doubting the information given. You don't have to tell them what they need to know, but if the in-game logic gives them a reason to doubt, they won't need to metagame it.

• A GM helps keep metagaming to a minimum by rolling all dice that might include hidden knowledge behind a screen, and to randomly roll dice for no reason, so they don't even know what you are rolling for all the time. The players can't cue their behavior off a roll they cannot see. There are all sorts of differing game and GM styles, but human psychology is what it is. Information known is used by the consious and subconsious mind. GMs that burden their player with extra reasons to metagame are just making immersion more difficlt.
• Lastly, reinforce roleplay and immersive play regularly and consistently. When the players stay in character, reward them. I actually hand out 'roleplay experience' at the end of every session, and it is scored on how well they stay in character and extras for especially character-building scenes. If this is enough to make a difference, they will start working towards the behaviors you are shaping.

I really hope that made some sense, and that some of it is helpful!

Answer 4

Metagaming

Gaming the game is one of the most popular yet pernicious practices within the hobby. You can metagame the rules, the plot, the party, and/or the GM. Here's how to try to avoid each of those.

Step one is just say "no metagaming please." Discuss with the group the various differences in play experience metagaming gives and get people to try not to do it. Often people are just doing it thoughtlessly because it's a "casual" game but usually respond fine, in my experience, to a general group agreement to try to not do it.

In the end, players often don't want to metagame but feel forced into it at times. Once you get people to generally agree they shouldn't be metagaming and they're not just doing it casually, most of the remaining problem is when they feel like metagaming is necessary for success - whether that's survival, or getting XP, or just moving the game along. As much as you can remove the temptation by not making it necessary to metagame, the better.

Metagaming the Rules

Players allegedly don't know the game's rules, although sometimes there's a fine line if the rules basically create the physics of the world - if one sword hit just never kills any experienced person, then it's only arguably metagaming for the player to scoff at being held at swordpoint and just say "I soak up the AoO." For fundamental stuff like that, you have to either change the game system to a decently realistic one, or fudge the system with ad hoc rulings to value game world realism over "what the rules say" at a given time. I like to deliberately add some uncertainty - No, you can't just place a fireball automatically on these squares and not have a chance of getting your colleagues on adjacent squares, you can roll for it though...

A more common part of rules metagaming is the "I know all about that monster even though I'm a novice because my player does." You can help this by not giving out monster names but just describing them, using variant monsters, and frowning on metagaming explicitly at the table. This also comes up in genre games (this is Star Wars, so I know that X is in this location). Here, GMs should mix up subverting expectations (not in MY Star Wars universe) with general encouragement (with positive/negative reinforcement if you want).

Metagaming the Plot

I find this happening to me more as I get older and I've seen all the "clever twists" your average adventure author brings to the table. In one campaign, we got rumors of a space pirate called Rojo Beluga. As a player I quickly understood that "Rojo Beluga = Red Whale = red herring = ignore these rumors." You get fed stuff that is obvious traps, or obvious diversions, and you metagame to bypass them (although again here there's a fine line - characters can smell BS on a too-pat damsel in distress story too). In fact, in many games players feel obligated to metagame to take the weak ass story hooks presented to them because otherwise the story won't progress - which is fine but once you've broken that fourth wall it gets easier to breach every time.

To combat this, don't be cutesy. Think through plot hooks and elements as "would a reasonable person - and/or my PCs - go along with this?" And then sometimes you're going to think you're being clever and you're not, and they thwart you - take it in good grace and go with it. Forcing the issue then puts them into metagame mode.

Metagaming the Party

Everyone knowing everyone else's stats and backstory and inner thoughts creates a metagame clique. Don't let people share character sheets or use "rules talk." Pass notes and use information compartmentalization for things not everyone knows. This ends up providing a pleasingly realistic interpersonal dynamic that then has intriguing depth to delve into, instead of everyone being a known measure and open book.

Metagaming the GM

Often results in one of the above kinds of metagaming, but derives from the players "knowing you." "He always puts in a dragon." "He just bought this supplement and has been babbling about how cool witches are, expect one this session." "Any female NPC he runs that won't sleep with you is an assassin." If you are lame and predictable, then it becomes easy to metagame you. Don't telegraph and change things up. When possible go for the messy diversity and realism you can find in the real world.

Answer 5

The Simplest Solution

Tell them when the encounter is over. Then don't let them try to revisit that encounter.

Geting there in the first place

In order to have it be an issue in the first place, the players have to have been rewarded for it at some point.

Several techniques lead to this perseverant behavior:

• allowing multiple rolls to find things
• having every encounter be worth XP
• Only giving Encounter XP if the players find the macguffin
• fudging die rolls.

Getting away from it

Adopting the following table conventions really helps:

• Let It Ride
• Dice are Players, too
• Bonus XP for not using Player Knowledge
• Low-XP encounters
• Color Encounters
• Play the Psychology of the Character
• Non-XP rewards

In detail...

Let It Ride

Nothing gets rerolled unless the rules specifically call for it. The first game with such a rule I encountered was MegaTraveller, tho it wasn't so explicit; Burning Empires was my first contact with it as an explicit rule.

A Roll to find something, for good or bad, convinces your character of the existence or non-existence of that thing. Fumble a search roll for a non-extant secret door, and you're convinced there is one that you don't know how to open.

Dice are Players, Too

This is an old concept, but a new wording for me. The dice are there to add something to the game. Accept their input, as it leads to a better game most of the time. If you go to the dice, always be willing to accept their input; corollary - if you have reservations about the outcome, don't go to the dice.

Bonus XP for not using player knowledge

The moment you award someone publicly for not using Player knowledge that the character shouldn't have, you start to break that reward cycle. Unfortunately, this one needs to be VERY consistent, due to the "random reward effect"... It need not be much, but it should be just enough to be noticed. In BXCMI D&D, I considered this "half a bonus" (an XP bonus is 1/20th of a level per the Cyclopedia).

Low XP Encounters & Color Encounters

If you put in encounters with John the Crier, and he's not a threat, has almost no useful information, but adds color and verisimilitude, players get used to these small guys. If you reward interacting with them, players will tend to do so.

For 1st level D&D, a 10 XP encounter isn't much, but they add up, and having several quick, colorful, low XP encounters can make a game much more story and much less tactical wargame.

Likewise, not all scenes need to have something to do. It's fine to occasionally describe a room just to set the feel of the place. For example, if the entrance to the dungeon is a crystal staircase down a hole, it's a huge difference from a steel one, or a hewn rock one. Describe the nature of the mold on the walls, and the moss by the door. Don't waste time, but do add just enough to set a tone in these color encounters.

Likewise, some encounters are with intangibles. Arriving in town, and everything reeks of an acrid scented effluent, instead of the normal sulfurous effluent, describe the difference. Nothing to do about it, but it provides information... "something's amiss here, because the sewage smells wrong." Giving it as one line like that is a clear warning. "Ah, the smell of the city. Baking, tanning, effluent in the open sewers... There's an acrid tang, and the sewage isn't quite as rotten-egg a smell as one woud expect." (I'd be worried, because the implication is either dysentery or vomiting... if you can smell it in the sewage, in large numbers.) Allow the knowledge rolls to understand it, and move on. If they don't make, just move on.

Play the Psychology of the Character

When a player decides a character has some psychological element, reward playing it. Don't punish non-play, but reward the play of it. Sometimes, this means reducing color text to simple narrative, as the group doesn't care, but the player knows the character should.

For example, if playing a merchant in a tunnel-questing game, reward the player for "I'll check the local assay office for prices", while in a merchant game, the player might need to actually spend time interacting to earn the same bonus.

Likewise, if the character "has no interest in frogs," and ignores the Frog God statue of Gold, reward the player. Sure, it's valuable, but the character doesn't care, so leaving it be, especially when this results in not finding some clue, is worth reward.

Non-XP rewards

Not every reward needs to be experience points. Character cash can be a reward. Magic items. In games that have them, fate points or hero points.

Heck, one of the best rewards is a small token food reward. I once experimented with rewarding players for clever actions not with Die-Roll mods, but candy. Got some really clever actions.

Even a "Retry Token" allowing retrying in the face of Let It Ride is a good reward. (Especially in systems with explicit LIR rules.)

Answer 6

As a trivial random encounter is winding down, a player thinks "That was too easy... The GM wouldn't give us an encounter that doesn't give XP," and starts searching for hidden bad guys or other clues.

The trivial encounter winds down and a player looks for the other shoe to drop; have something happen to one of the characters (snake bites the Wizard while the priest is looking for the big baddie, etc.) Trust me. If they are playing their characters, there will be a "and where was Mr. God's gift to the Clergy when I was suffering from this snake bite?". But be careful. This is also good tactical thinking. 1 bad guy who is almost dead, and the rest of the party has the baddie surrounded, but the missile fighter does not have a clear shot. He searches for a new target both to be effective, but also to be able to shout a warning to the rest of the crew. Oh, and throw more trivial encounters at them. They will get used to the occasional (or frequent) trivial encounter as part of the norm and stop meta-gaming as much.

There are four statues in a room, and you use miniatures to represent them. The players think to themselves "Well if the GM used miniatures, these things are definitely coming to life," and get ready to fight.

Two options:
1. put d20s down instead of minis for the statues
2. have a significant story event happen where the players drawing weapons would be a BAD idea.

When you lay down a map of a seemingly random part of the forest trail, the players think "This must be important, let's search around / prepare for battle."

Simply, do this when it is not necessary. The crew stops for lunch? Have them map it out. If they stop and eat on the road, have a merchant's train come through half-way through their meal. If they pull off in a field, have a fellow traveler stop and ask them to play a friendly game of [sport].

You roll a 1 for a player's knowledge check and feed them inaccurate information. They saw the roll and know you're full of it.

Tell them to knock it off. It was suggested in an answer to one of my questions (or was it a competing answer to one of my answers?) that you roll the rolls in the open and then the challenge is to play what the numbers show. Sounds like fun, but if the player is meta-gaming, they will lose the fun for the game.

A final note: Some meta-gaming will happen. You can't stop it, and I would not try. Sure the player knows the fire demon is weak against frost damage. The player may or may not, but IRL, I've written some code and looked back and said words to the effect of "how did I come up with THAT solution?" It was "out of character" for me, but solved the problem at hand more efficiently than I would have otherwise. It's part of the "on the job training" process. Try something odd that is out of what you would normally do, survive the encounter and remember next time the fire demon comes around to grab the frosty axe.

Answer 7

Use their assumptions against them. Make fights that are underpowered. Put statue miniatures on the table. Draw maps just to show off your artistic talents. Start encounters without maps and let the players wander into them without being ready for a fight. Give them correct information on a natural one and laugh with glee as they assume it's wrong.

Or just roll with it. Some players really like providing a director's commentary of the entire game. It's not great RP, but not all the players are there to see things through their character's eyes.

Answer 8

My advice is to keep changing things.

A trivial encounter can just be a trivial encounter now and then. Let them have one when they are under time pressure so they miss something they want if they do not hurry.

If they know that you only put a map down when an encounter is serious, do not give it to them until after it has started, or give them maps randomly.

Make all knowledge, observation and similar checks behind a screen and tell them the results.

Think what assumptions you would make as a player in the same situation and allow those assumptions to backfire on a party.

For example, I once had a Cthulhu game with a vampire bad guy. The players had lots of clues about how to stop him and what his vulnerabilities were but they went in with crosses and opened his coffin ready with a stake. If they had listened to the clues rather than remembered the films they had watched they would have known that a cross would not bother him and although he rested in his coffin he did not sleep. When they opened his coffin up they were surprised to find him laying there, awake, with two pistols. He started shooting at them and killed two of them before the remaining four could get away (one almost dead and being carried).

Answer 9

I think the simplest thing to do is to remind your players that they are playing a character. That they should abandon their own thoughts and think about what their characters would do. Your group's fighter is probably smarter then the character they are playing, but they aren't nearly as strong. Encourage that player to remember that while he isn't dumb as a rock, he is the least intelligent member of the group.

Fake outs are a bit important here too. Make sure that every once in a while the statues don't come to life, or they make it across a map you put down with nothing happening. The sense of foreboding the entire group will get will be amazing. I think unpredictability here is your friend. Your players probably are fairly familiar with how these things tend to work (general gaming/fantasy tropes), make sure that sometimes they do not work as expected.

Answer 10

One has to be very careful here, while much of the above is true, let me tell you a little tale. In a game I played in years ago our party was escaping a camp where some weird and deadly things were happening. One player was well ahead of the group and came to a road in the wilderness with chasm just beyond, he could go left, right, or turn back. The GM did not pull him aside and we all heard this exchange and heard him chose right. A bit of the way down the road on the right path he faced something which would have killed even our full party and his character died. Now when the rest of use reached the road we were faced with a dilemma. If we chose to not take the right path it would could seem like we were using player knowledge, no matter how we came to that choice. Someone suggested (as many here might) that a random dice roll could solve the issue. But when you are faced with two paths is your choice always really random? Do you look at he state of the road in both directions? Is one more traveled? Do factors like lighting, the density of the brush, or animal tracks play into your choices? But at that point even if we wanted to say were were making use of those factors, if for any reason we chose left there could always be the lingering question of the use of player knowledge.

I put this here to suggest you tread carefully and ask yourself if you have overtly or even accidentally given anything away before you come down on your players with false information, red herrings, or punishments unless they are being painfully obvious about player knowledge abuses.

Answer 11

While there are some good answers here I wanted to add my experience into the mix.

What is metagaming?

(I feel really strongly about this). The community have over the years co-opted what the term Metagame means. Inherently Metagaming (per its definition) is what game designers use to ensure the rules of the game are balanced and fair. Meta gaming deals with advance strategy and, most importantly for the playing group, metagaming forms part of the inherent social contract between players and GM: that the game is a shared, noncompetitive experience. The players work together.

The GM presents obstacles but isn’t actively invested in the players’ failure. And so on. The interactions at the table and the social rules that govern them? Those are part of the metagame.

Answering using 'Metagame' as implied by the question

Now for the answer: I will use the phrase Metagame in the context that the community has decided to use it for: an idea that a player's decisions for their character are made with some outside non-game knowledge and therefore spoil the game.

Your question makes an assumption that the situations you describe are either a problem, or reflect a behaviour the players need to change. I would challenge that by saying that each of the issues you describe either isn't an issue at all (in anyone's other then a DM's head) or is a situation that is brought about because of the DM.

As a trivial random encounter is winding down, a player thinks "That was too easy... The GM wouldn't give us an encounter that doesn't give XP," and starts searching for hidden bad guys or other clues.

Investigating an area after a combat is not metagaming. The whole idea of an adventurer, early on, is that they survive off the land, by finding anything they can make use of. Corpses are looted, rooms searched, etc. Expecting your players not to search after an encounter because in your head it is random makes little sense in that context. Do you have to provide something if there is nothing? No. I have had players roll a 20 for investigate after killing an enemy, they find no more then they would have if they rolled the DC of 8 I had set in my head, sometimes all that is ther is a few pinecones and a half-eaten deer. If players are constantly looking around, hunting for something, that suggests to me as a DM that either my encounters are not actually delivering anything of any value to the characters, they are not learning anything here, or the players themselves are getting bored. Any of that suggests that as a DM I need to change my behaviour mix things up a bit, or just stop rolling the random encounters table.

There are four statues in a room, and you use miniatures to represent them. The players think to themselves "Well if the GM used miniatures, these things are definitely coming to life," and get ready to fight.

Again is this metagaming? Adventurers live their lives on the edge of combat, expecting danger around every corner. They live with a knowledge that at anytime anything could be waiting to kill them. It is why we allow them to spend their entire life stealthing everywhere, checking for traps every two minutes. As a DM I don't accuse my players of metagaming when they tell me I stealth down this cavern, or I stealth through this wood. I accept that as part of the Darwinism that is being a successful adventurer - the characters "in-universe" would be very cautious.

Your players enter a room with 4 statues. You make a point of describing them in detail which means that the characters have really taken in statues as being of interest which would instantly make the adventurers feel concerned. These are different, unusual; it is perfectly believable that, by being so special, they warrant the players taking a close interest. The characters would be wary, they know magic happens, they search for traps constantly, so these statues might be a trap. Expecting players to behave differently means for them to play their characters as suddenly ignoring their environment.

But as A DM I want to provide a shocking surprising moment, I want this to be unexpected.

There are plenty of ways you can do this. The first way is to put statues everywhere and put them on the map, describe the first, normal stone ones in detail as part of your description of the background. The characters will just see everything as just another statue. They may still approach every statue with caution but this is what adventurers do if they intend to survive; it isn't necessarily the player acting against what there character would do. Using that kind of description will also train your players that things put on the battle map are not always more then "just a statue."

The other way: don't put the statues on the map until they come to life, describe them when they enter the room but, if you don't usually place statues on the board don't do it now. By putting them on the board you make into items them of interest to the characters. Why? It is a part of the environment that stands out. You are telling your players, as part of the story, that these things here are important. Expecting characters to wander around aimlessly and ignoring the statues until "something happens" is asking them to act against their own motivations as adventurers who face danger and magic as a matter of habit.

When you lay down a map of a seemingly random part of the forest trail, the players usually think "this must be important, let's search around / prepare for battle."

Much like the point above, the characters are always on alert because that is how they stay alive "in world." By laying down the map you have now indicated to the players where the story is, but, as characters you can assume that they are constantly on alert. This is not meta gaming, it is being in-character. We skip large parts of our players' journey because "nothing happens" but, I never assume that my players are just strolling without a care in the world.

Travel through a Roleplay universe is made cautiously, carefully, always on the look out. If it wasn't we wouldn't bother asking players to roll perception rolls to see if they spot that person following them. All you have done is told your players that travel is now going to involve something, but you do that with or without a map the moment you say "after several hours of walking ..."

A useful DM tool

A simple way to stop the feel that your players are reacting to the story and not their characters is to insist on them defining patrol patterns: what is the order of the party as they move through the environment? What are they doing, how far are they from each other? Maybe get them to agree to a defined formation that is the standard if they haven't said differently. This way when the encounter starts you don't need to get them to think about how they are set up. For you, the DM, never see character prepardness as being more then simply what they would do anyway. The question out of the game is when you as a DM ask them to define what they will be doing.

You roll a 1 for a player's knowledge check and feed them inaccurate information. They saw the roll and know you're full of it.

Personally as a DM I don't ever roll for a player. They make make their own rolls, with the exception of passive perception. To get around that I just roll dice randomly all the time as a DM. But, if the roll is failed I never provide false information; I tell the player they don't know, or they don't see.

If you want to do the above, make your rolls in secret so they can't see. The same can be said for players rolling to spot a trap and rolling really low. There may be no trap there but they know if there is they haven't found it. On this I expect my players to behave accordingly, open the door anyway, step down the corridor. If they don't then I will, out of game, have a small chat with the player.

Even the most disciplined players sometimes will take advantage of "Player Knowledge" over "Character knowledge." Sometimes it's hard to even tell the difference. You know that you should switch to your frost battleaxe against that Fire Demon, but does your city-raised fighter with the intelligence of an old boot know?

In this specific issue knowing you're facing a fire demon means you probably know that he isn't going to do well against fire based attacks and maybe lets try using a cold weapon. But this situation is also the one that player are in a lose-lose on. As a DM what do you want your players to do? Do you want them to hit the fire demon with a ton of different attacks until they "stumble" on the one that works? If the first thing they use will the DM accuse them of metagaming it, should they roll a dice to randomly decide attack by attack? Doesn't that strain credulity a bit?

Also while a players intelligence may be low, or they may not have faced this enemy in this campaign before, the world of fantasy is full of tales, stories, and history. If you compare it to the ancient world the ancient Greeks told stories by word of mouth yet, I imagine they all knew how to solve the riddle of the Sphinx. It is perfectly acceptable to believe that one of the characters has some knowledge of a fire-based being and what its weaknesses might be. They might remember that famous fairytale where a prince killed a fire creature with cold, or might have been sat in an inn while an adventurer regaled them with stories about a fight they had in the fire realm. As a real person I am full of bits of information I have picked up over the years, stories I have heard. It is perfectly reasonable to accept that your characters know more then you expect them to without asking them to explain how they know this thing. Remember most characters have had a life before this campaign, for some races that life may have been many decades long.

What strategies are there to discourage the use of player knowledge, and how can you cut back on perceptions that players have of the genre itself?

In conclusion I don't think you should. This is a step toward adversarial GMing.
Players characters are not wide-eyed and new to the world; they have not been created just seconds ago (unless you're playing Paranoia but then all your problems go away). Their knowledge and ability goes beyond a simple dice roll or stat. The most stupid character in the game can have experience to draw upon, or the ability to foresee that something might be about to happen.

Anyway: do the examples presented here really break the game or affect enjoyment?

They might make a DM work a little harder, or think a bit more sneakily, but that is never a bad thing. Now if a player out of character realises that person x is the bad guy and just kills them with no in game evidence, that is a problem that needs discussion but none of the examples you provided above, in my experience, are.