Mouse Guard Roleplaying Game on Amazon
What Mouse Guard is all about: fighting for -- and challenging -- the characters' Beliefs. Beliefs are a stat in Mouse Guard, and with good reason: they're at the heart of the game.
Beliefs do several things. They give you an easy roleplaying hook -- Mouse Guard mice have a code, and upholding that code in each mouse's own personal way is a core element of the game. Your character's Belief also signals to the other players -- and the GM -- what you're interesting in exploring during play. For the GM, challenging Beliefs is a great way to get a player involved (and part of your job). And Beliefs are one way to earn rewards (XP, essentially), in the form of Fate and Persona Points.
Beliefs need to be general without being too general, and strongly expressed -- they're about getting you to make interesting decisions. Here's a sample belief from p. 43 (for Saxon, a character from the comic):
"The best solution is always found at the point of my sword."
That's excellent roleplaying shorthand -- even if that's the only thing you know about Saxon, it tells you a lot. As the player, you can and should fall back on your Belief when deciding what to do in-game; you'll be rewarded for playing it, as well as for playing against it when the circumstances warrant. As the GM, you should challenge the PCs' Beliefs in play.