While in game, open the pause menu, hold the right shoulder button and enter the following codes: (Note: Does not disable achievements and must be done in single player!)

GI John Doe Mode: Left Thumbstick, Left Thumbstick, A, Right Thumbstick, Right Thumbstick, B, Left Thumbstick, Left Thumbstick, X, Right Thumbstick,Right Thumbstick, Y
Super Ragdoll mode: A, A, B, B, X, X, Y, Y, A, B, X, Y (Fun Physics)
Third Person View: X, B, X, B, Left Thumbstick, Left Thumbstick, Y, A, Y, A, Right Thumbstick, Right Thumbstick (Warning: Reports of bad camera for this cheat. Use at own risk)