How to unlock completion bonuses
Finish the game under easy difficulty to unlock the special menu and movie mode.
Finish the game under normal difficulty to unlock two player mode. In this mode, one person moves using a Dual Shock 2 controller, while the other shoots with a GunCon2.
Finish the game under normal difficulty with a rank "B" or greater without using any Resurrection Packs to unlock infinity mode.
Finish the game on Normal without using a resurection pack and get at least an overall B grade at the end of the game.
Finish the game under hard difficulty to unlock the game art.
Shoot some credits
During the credits you can still use your gun for a little more fun. Or to get your aggression out since the game is over (too short).