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If you've played tabletop games for a while you'll have likely come across a player who meta games. Whether this is a bad thing or not depends entirely on what kind of player or game master you are and on what the rest of your playing group wants. The majority of people dislike meta gaming, at least to some extent, and will try different ways to either get rid of it or to minimize it in their game.
Fortunately there are plenty of ways to deal with meta gaming and that's what this guide is intended to help you with.
But first, what is meta gaming exactly? Meta gaming is making character decisions using knowledge you, as a player, know, but your character doesn't in a way that makes the decision the character makes completely unrealistic. Here's an example to illustrate it:
Say your character faces a particularly powerful warrior. Based on the description the game master (GM) has given you you know this particular warrior has a weakness: their Achilles' heel, literally, their Achilles' heel will be their downfall.
Now you as a player will know this, but if your character has never come across this before there's no way your character would know the Achilles' heel of their opponent is their one weakness. If you state your character is attacking the Achilles' heel anyway you're meta gaming.
Some of you might consider min/maxing to be meta gaming as well. Min/maxing is maximizing one or several skill point and talent trees you have to create the most powerful character you can using every detail in the rules, while forgoing other elements. This is not meta gaming. As long as your character doesn't act with knowledge it couldn't possibly have and as long as everything you've created is within the rules you're not meta gaming.
However, min/maxing can also come in the form of trying to make every action the perfection action. Kill that monster in such a way that the most precious parts can still be sold or using every trick in the book to overcome a specific obstacle. This could be seen as meta gaming, but if it fits the character it could just be role playing as well.
Of course, that doesn't mean it can't be tedious as well and it's often a delicate subject to tackle in some groups. But since this is a different beast entirely I've covered this in a separate guide found here.
You might wonder what the big deal is. In video games you pretty much meta game all the time, why can't you do it in tabletop games? Well, you can, but it'll depend on the people you play with. It's a group game after all.
Most parties don't enjoy meta gaming for the simple reason that it breaks immersion and defeats the point of the role play element in role playing games. But if your party is happy to meta game then by all means meta game full speed ahead.
Now before I delve into the ways to combat meta gaming let's first make one thing clear: Meta gaming can't be avoided completely and to some extent shouldn't be avoided completely (more on this later).
At the end of the day we're all playing a game to have fun and mistakes happen, players might forget they're not in the same room, they might have forgotten their character wasn't present for a certain story detail and so on, but that's okay. A quick pause in the game is usually enough to deal with this and it shouldn't get in the way of having fun.
Right, let's delve into the ways to combat meta gaming. Obviously meta gaming is entirely based on knowing what will or might happen, so the obvious solution is to limit how much players know.
If you're running a module make sure your players don't know it inside out and don't have access to it, if there's a monster manual for your particular game make sure your players don't read through it all and memorize every monster and so on.
Dice rolls could also remain hidden. If a player fails a stealth check for their character they often try something else, but realistically their character might not always know they failed until it's too late. So hiding dice rolls could help prevent meta gaming as well. Of course rolling your own dice is part of the fun, so this option isn't always viable.
Obviously all of this isn't something that can be prevented entirely. One way or another players will likely end up with knowledge of elements of your campaign. Some monsters are just common knowledge, like vampires and werewolves for example and some elements have been done in so many other stories that many people can spot them from miles away.
Fortunately this doesn't mean there's no solution, which brings me to my next point.
Customize the monsters, customize the spells, customize anything your players might have prior knowledge of. Customize everything. As soon as you change how something works your players will no longer know the exact ways to deal with it and will have to figure out a solution, just like their characters do.
You can customize anything you want and the changes don't have to be huge either. Does a specific monster enjoy shooting icicles and would this description give away what it is? Maybe this version shoots discs of ice or bolts of chilling energy. Anything that'll make the players go "Huh" will work.
Of course the easiest ways of making sure player knowledge can't be used is by changing the facts. Werewolves are vulnerable to silver bullets, right? Nope, these versions are vulnerable to chocolate pudding. "What? That makes no sense." you say. It might, as long as it fits within the story. Which is an important thing to remember. It's perfectly fine to alter information so the players won't know what to do, but it shouldn't be impossible to figure out what a monster's weakness might be.
Allow your players and thus their characters to do research within the world, perhaps witnesses know of something or maybe there's a dusty, old tome in a library. Figuring these things out will make the entire campaign more satisfying.
Changing the pace of your game and play style can help prevent meta gaming as well. To a large extent this could be seen as both part of the customization and the knowledge points I just made, but it's a very effective way of preventing meta gaming.
Say you describe a scene as follows: "You tread deeper and deeper into the forest, the sounds of nature are all you hear as the sounds of civilization are drowned out. Suddenly you see a child in a small clearing." If at this point you grab a monster manual or other papers your players will automatically assume combat is about the start and may play their characters more cautiously. Realistically their characters wouldn't assume a child in a clearing would automatically mean a combat situation, so this could be meta gaming.
So not grabbing the monster manual would be the right option, right? If combat is about to begin, yes, definitely. But it could also be used to fool your players. If you grab your monster manual every time you introduce a new NPC your players won't be able to use this as an indication of impending combat.
Grabbing a monster manual is obviously only 1 way of tricking your players, but the overarching trick is to prevent players from knowing what happens based on what you do, this way they have no choice but to find out through playing their characters.
Most gaming systems will have some form of skill check system in place to ensure the dice or some other element decide the outcome of the story, so make use of this. Many people only make their players roll skill checks when they want to overcome an obstacle, fight a monster or interact with an NPC, but they often forget that having and recollecting knowledge of monsters is also an obstacle.
Consider the following: Say you're out in the woods and suddenly a bear attacks you. What are you supposed to do? Some of you might know the answer, some might think they know and some have no clue. This is the same for the characters you play, which can be replicated to some extent with a skill check.
Make sure the skill checks reflect what the characters could realistically know though. A ranger would likely be able to figure out how to handle most forms of wildlife, but an underground worm-like monster is a whole different story.
This system encourages role playing to some extent as well and at the very least might encourage some more (background) story opportunities. When the player characters are browsing a library to find a tome with information on a monster they'll face or when they're interrogating the villagers for more information there's plenty more other information that could be given, perhaps even side quests.
Some game masters might be inclined to punish those who meta game, but I urge you not to. In the vast majority of cases it'll simply ruin the game, either sooner or later, and end up with a wasted opportunity. Instead it's better to either get rid of the meta gamer, assuming they're not willing to stop it and thus ruin the fun for everybody, or work with the meta gamer(s) in some way that resolves the meta gaming in the first place.
Having said that, it's entirely up to you and your group to make sure your game goes the way you all like it. Compromises may need to be made, but it's unfair to expect a player to completely change their play style without helping them.
If it does turn out to be impossible it's not the end of the world to change up your group, in the long run it's often for the best.
No matter what kind of meta gaming you're dealing with or in fact what kind of player you're dealing with, communication is key to resolve whatever issues you face. If you don't talk about what you expect from your group and the way they play, if they don't convey what they expect from you as a game master and if issues aren't brought up you're bound to end up with frustrations and sour game sessions. So talk things out and compromise where needed. Remember: it's only a game.