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, 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 to force them to do something.