After beating the game, or at least a level, you'll notice that you'll have a total of five unlockable cheats when attempting to replay a level via the "level select" option. Here's how to unlock them.

Unlockable	How to unlock
Bullet Cam on Every Kill	Find every clue.
One-Hit Kill	Beat game on Hardcore with Free Aim.
Unlimited Ammo	Collect all golden gun pieces.
Unlimited BulletTime	Earn a gold-medal score (about 7,000) or better on every level in arcade mode.
Unlimited Pain Killers	Beat game on Hard with Free Aim.