Cheat List
You must first finish all mission objectives in the single player game before the cheats become available. Then, during gameplay, press "back". After that, ress the following buttons to activate the cheat:

High Pitched Voices:X,A,Y,B,X
Big Heads: A,X,B,Y,A
Chicken Explosives: X,X,Y,A,B
Big Bodies: B,B,Y,X,X,A
Slow Mo: Y,Y,B,X,A
Paper Mode: B,A,X,Y,A

Unlock Characters
J. Stone - Finish Special Objective in Campaign Mission 4 on any difficulty
S. Grey - Finish Special Objective in Campaign Mission 5 on any difficulty
B. Gordon - Finish Special Objective in Campaign Mission 6 on any difficulty
W. Jacobs - Finish Special Objective in Campaign Mission 1 on any difficulty
H. Ramirez - Finish Special Objective in Campaign Mission 2 on any difficulty
K. Henkel - Finish Special Objective in Campaign Mission 3 on any difficulty
A. Galinsky - Finish Special Objective in Campaign Mission 1 on Recruit during a Quick Mission in Mission Mode
N. Tunny - Finish Special Objective in Campaign Mission 3 on Elite during a Quick Mission in Mission Mode
L. Cohen - Finish Special Objective in Campaign Mission 4 on Recruit during a Quick Mission in Mission Mode
D. Munz - Finish Special Objective in Campaign Mission 2 on Veteran during a Quick Mission in Mission Mode
S. Ibrahim - Finish Special Objective in Campaign Mission 6 on Elite during a Quick Mission in Mission Mode
G. Osadze - Finish Special Objective in Campaign Mission 5 on Veteran during a Quick Mission in Mission Mode

Cycle back through weapons
To move back through the weapon selections, press Y.

Boost stats
Start a private game in cooperative mode. Play the game on the elite difficulty setting with unlimited respawns and grenades. You will get a win and 45 kills easily.

Single mission objective 17, Swamp Airfield
Every time you kill a group of enemies, save the game. When you get killed, load the saved game and kill the person that got you.