Select "Options" at the main menu, then select "Game". Press Right(2), Left, Right, Left(2), Right at the Game Options menu. The screen will return to the main menu to confirm correct code entry. You now have access to the Debug Game mode, where you can pick any level/mission and select any scene after the major loading scene of the mission. As well as Debug Game mode, if you go back to the options menu after the code enabled, and before you start a game, a new "Cheats" option is now available. The "Cheats" menu has options for "Unlimited Ammo" and "God Mode". After activating either or both options, you can start a new game, continue a game, or play Debug mode.
In Debug mode, once you complete a mission, you go back to the mission selection screen. The mission you completed is now checked. If desired, you can start from where you originally left off or you can go back and enable the cheat again to unlock Debug mode once more and continue where you left off. Note: Once a debug mode mission is complete, you will start back where you originally were at, before the game save. For example, if you only completed mission 1 but you used debug mode to complete mission 7, then mission 7 is complete, but you will start at mission 2. You cannot go past mission 2 without completing it or redoing the code for debug mode.
Complete all of the secondary objectives for a mission to unlock the bonus gallery pictures for that mission. To see the pictures, highlight the yellow tack at the mission selection screen, then press X.
Always use the knife. Silent kills work best and will not attract other guards.