On any level, in any game mode, pause the game and input the following codes.

up, down, left, down, left, right. --> Funny captions will appear when you kill rebels.
up, down, left, down, down, left, down, down, left, down, down, down, left, right --> Infinite Ammo
up, up, up, left, down, down, down, left, up, up, up, left, right --> Invincivbility