It is important to remember that the player and the character are two different persons. The more the two are kept apart, the better your games can be.
The most obvious example of this is the dice rolls you make. All dice rolls are called “game mechanics,” as are other details such as Armor Class, Hit Points, and so forth. These things would not be part of a character’s knowledge. The characters would talk about armor, health, and attacks, but never about Ability Scores, Hit Rolls, or other parts of the game. Everyone can usually tell whether the players are in the role of the characters or being themselves, handling game mechanics.
If there is any doubt, the player should clarify.
Suppose that a player has a Chaotic character (unusual, but not impossible). The character will act wildly at times, but the player should remain calm while dealing with the others. If the player acts Chaotic, the game will become very confusing and less fun.
There are many ways that this can affect the game, especially when a player knows something that the character doesn’t. For instance, if a character starts to fight a new, unknown monster, and the DM says “It hit you; save vs. Poison, please!” then all the players know that the monster is poisonous. But the better players will ignore that information. Their characters don’t know about the poison, and shouldn’t use that “player information.” The characters will probably find out after the battle, when they talk to their wounded friend and discover that the wound “stung, like poison,” or is discolored. (All of such descriptions are left to the DM’s imagination.)
A question like “What time is it?” could be answered two different ways: in “real time,” which any player can find by looking at a clock, and “game time,” of which the DM should be aware, but of which the characters might only have a general idea. The DM could reply, “Real time, it’s 7:30. Game time, it’s past noon, but you are not sure exactly.”
When the players remember the difference between themselves and their characters, everyone can have more fun in Role Playing. If a character has low Intelligence and Wisdom scores, for example, and then does something stupid, the player can honestly say “I was playing my character,” and others should remember not to get mad at the player. Their characters may indeed get mad at the stupid character, but it is very important to remember that the player is a different person.
This should not be used as an excuse for bad or selfish play. The DM should watch all the role playing closely to help everyone have the most fun. Since the object of the game is to have fun by playing roles, stupid or weak characters can be as much fun as smart, powerful ones - if the roles are played well.
The Dungeon Master may say, at any time in the game, “You don’t know that!” or “You wouldn’t think of that.” Good players will learn to avoid this type of problem by keeping the character knowledge and player knowledge separate.