Cheat: Third-Person View
To go into third-person mode while playing a single player campaign, hold down the right bumper and insert the following code: X, B, X, B, Left Analog Stick, Y, A, X, B, Right Analog Stick. If done properly, a message will pop up to inform you. If you want to turn off third-person mode, simply insert the code again.
Hint: Designated Killer
Get the full value of enemy kills in Story Mode and Terrorist Hunt by designating your targets. Designated targets will earn you the same EXP as if you killed the enemy yourself (instead of one-half).
Hint: Extra Magazine Glitch (Realistic Difficulty)
Ammunition becomes more of a problem on Realistic. If you see a dropped weapon that's the same as one you are carrying, reload your weapon first, then trade for the dropped weapon. Trade back your weapon from the ground and you will find your reserve ammunition increased by one or two magazines (depends on the weapon).
Unlockable: Lasers Instead of Bullets
To shoot red and blue lasers instead of bullets (G.I. Joe style), pause the game, hold the right bumper, and insert the following button combination: left analog stick, left analog stick, A, right analog stick, right analog stick, B, left analog stick, left analog stick, X, right analog stick, right analog stick, Y. ("[direction] analog stick" means you should press the corresponding analog stick in, as a button). This can also be done while playing online, but the game's host must insert the code. No other player needs to worry.
Cheat: Stronger Ragdoll Physics
To enact stronger ragdoll physics in your game, pause your game and insert the following cheat (while holding the right trigger down): A, A, B, B, X, X, Y, Y, A, B, X, Y.
Hint: Getting ACE Points With Teamwork -- Your Teamwork
Your computer allies are great for taking out enemies for XP, but not for acquiring ACE points. Keep them back if you can handle a firefight by yourself, since enemies killed by your team will not add ACE points.
Players who are using Bishop and Knight will gain ACE points respectively in co-operative Story mode. Just remember to keep the CPU team away. This does not apply for Terrorist Hunt and adversarial Multiplayer.
Hint: ACE and XP Farming
Found a spot to gain ACE and XP easily? Maybe guys are constantly dribbling down the fast lines and you can shoot them easily, or a bunch of terrorists are conveniently bunched up so you can kill them with a sack of C4.
Simply perform the actions you want for ACE and XP, then before you hit the next checkpoint, let one of those wannabe terrorists kill Bishop or Knight, forcing you to reload the checkpoint. You keep the ACE and XP you acquired (just don't turn off your system) and you can re-do that section again for more points.
Hint: ACE Point Actions
You can view the required actions for ACE points when you pause the game. They are:
Assault ACE Points
+3 Kill an enemy or player behind cover with penetrating weapons.
+3 Kill an enemy or player with C4 (it must be C4)
+2 Kill an enemy or player who is using a mounted machinegun
+2 Kill an enemy or player with a frag grenade or breaching charge (i.e., something that's not C4)
+2 Kill an enemy or player who is using a riot shield
Marksmanship ACE Points
+3 Kill an enemy or player at long range (about the size of your targeting circle at its widest diameter)
+3 Kill an enemy or player when they are on a rope (fast rope or rappelling)
+2 Kill an enemy or player while you are rappelling
+2 Kill an enemy or player who is sprinting
+1 Kill an enemy or player with a headshot (any weapon, although a long distance headshot will result in a long distance kill instead)
Close Combat ACE Points
+3 Kill an enemy or player at short range (their height should be a third to half of the size of your screen on either standard or hi-def)
+3 Kill an enemy or player who is stunned or flashbanged (vision impaired)
+3 Kill an enemy or player by blind-fire (this range may be slightly more than the short range kill distance)
+3 Kill an enemy or player from behind (distance may change this to a long range kill)