Chaos Theory features refined stealth mechanics. In addition to the standard light bar, the game also features an aural monitor that measures the noise that Sam makes, along with the ambient noise of the environment. It is important for Sam to make less noise than his surroundings; otherwise, the enemy guards will hear him.
The AI detection has been altered as well. In former titles, after Sam left an area, the game would do a sweep of the previous area for all unconscious or dead bodies in a well-lit spot. If any were found, an alarm would be triggered. In Chaos Theory, the bodies have to be discovered by a patrolling guard or security camera to trigger an alarm. This enables players to leave bodies laying out in the open, and, as long as the player has eliminated all NPCs and disabled all cameras, no alarms will be triggered.
Being spotted by enemies will still trigger alarms, and alarms will still cause enemies to become more alert and combat-ready (such as causing them to wear ballistic vests and helmets). However, triggering too many alarms will no longer cause the game to end automatically. Even killing civilians or friendly soldiers may not cause Fisher to fail the mission, although doing so will at least cause Fisher to be severely chastised by his superior, and cost him significantly in his mission score as well as cancelling some mission objectives, such as tapping phone lines and locating covert listening devices.
Chaos Theory adds a combat knife to Sam's close-quarters combat abilities. Sam can use the knife in multiple ways, such as threatening an enemy during an interrogation, killing an enemy in close-quarters combat, or piercing gas tanks on generators to aid in his stealth operations. Also, it no longer matters what direction Sam attacks from when using melee attacks, nor does it matter if enemies are aware of his presence, as opposed to earlier entries in the series where he had to attack from behind or on the side, and the enemy could not be alerted to him in order to take them down in one hit.
Sam also has the option of using lethal or non-lethal force when ending an interrogation, and with his close-range attacks. As an expansion on Sam's ability to shoot while hanging upside down (introduced in Pandora Tomorrow), he can now choke down or break the neck of enemies below him. He can also pull people over railings while hanging off a ledge and throw bodies off of cliffs or over railings, even onto other guards. However, the ability to shoot around corners has been removed, but this is balanced by being able to switch the side of Sam's body the gun is on while in a firing position.
