NPC AI is determined primarily by the "Fear/trust system". The trust system determines whether the NPCs will follow Blake's orders and join him in combat. To do so, the player must ensure the NPC trusts Blake, and does not suspect him of being a Thing. NPCs have four levels of trust; red, amber, green and 100%. Red means they are convinced Blake is a Thing and will attack him. Amber means they are not sure if he is a Thing, and although they won't attack him, but they won't follow his orders. Green means they trust him and will follow his orders. 100% means they have complete faith in him, will follow his orders, and even if he attacks another NPC, they will support him. To gain the trust of NPCs, Blake can give them weapons and ammo, heal them, put himself at risk to protect them, or use a blood test kit on himself to prove he is not a Thing. Actions which deplete trust include accidentally shooting teammates, taking away teammate's weapons or ammo, or pointing a weapon at them for a sustained period of time.
The fear system dictates how scared a given NPC is. There are three levels of fear; normal, scared and "Crack-up". When an NPC enters "Crack-up" mode, Blake has only a limited amount of time to reduce their fear level, or the NPC will kill themselves, attack Blake and other NPCs, attack the environment, or die of a heart attack. Each NPC responds differently to different surroundings; for example, medics tend to scare easier than soldiers. Generally, however, fear increases when entering rooms covered in blood, entering dark locations, finding particularly disfigured corpses, being attacked by multiple enemies, and hearing enemies but not being able to see them. When an NPC is scared, it affects their performance; soldiers become less accurate, engineers take longer to repair fuse boxes and medics don't heal characters as quickly. Blake can reduce fear by giving NPCs adrenaline, topping up their ammo, moving away from particular locations, or successfully defeating enemies.
The game also features an infection system, which determines whether or not an NPC is infected by the Thing. Although most NPCs are scripted to transform into a Thing at specific points in the game, they can also be infected at any time prior to this. When the team comes under attack by enemies, the possibility of infection is based upon a probability system whereby any teammate who comes into direct contact with an enemy can be infected. Although infection does not alter their immediate behavior, it will result in them turning into a walker-Thing after a set period of time, at which point they will attack the team.
