They take AI and move them from one point to another, based on what's occurring. That is how they control the flow of the game. "If I'm an AI, fighting with enemies around me, one of the decisions I have to make is where is the most advantageous position for me to be standing? In each environment, there are hundreds of points specified by the level designer - these are possible places an AI might like to be. That is where the re-playability comes from." The Grunt will always run away, but you don't necessarily know where he'll run away to. We try to go for predictable actions but unpredictable consequences. It would be bad if they only ran away half of the time, because then the player can build a plan that will only work half of the time. For example, if I sneak up behind a Grunt and surprise him, I expect him to run away. The player can do things and expect the AI will react in a certain way. What you want is an artificial intelligence that is consistent so that the player can give it certain inputs. The goal is not to create something that is unpredictable. We don't do things by random chance very much.
They have access to all this game information and to all the numerical quantities that control the behavior of the AI. For example, at that top-most level, it should say, 'If there are enemies I can see, then I should be engaging them in combat.' But if the AI is a cowardly character, it might say, 'If I am faced with overwhelming force, then I will retreat.' That is the level where our game designers come into play. "So at every level of the hierarchical tree, those decisions about what the most appropriate thing to do is made based on the knowledge of what is going on in the world and knowledge about the type of character the AI is. There are six enemies and only two friends in here.' That is an example of more processed information that is used to create a more persistent knowledge about what is the state of the battle or what is the state of the world and how should I react to it?"
For instance, if I have memory of seven characters in this room and one of them is my friend and the other six are my enemies, I will have the knowledge that says, 'I am in a battle in close quarters with an overwhelming force. They take that memory model and turn it into more specific combat information. So they have memory models that are what they know about the world. But the AI would keep a memory of that character and the fact that they last saw her there, and when she left, she was moving in that direction. I see a woman wearing a black sweater right there.' You then turn that into more processed information by saying, 'I have a memory there that is who is in this particular state at this time.' If were to walk out of the room and go around that corner, I wouldn't see her anymore. So is able to say, 'At this moment in time, I'm able to see sitting right there. We take that information about what the AI can see right now and we turn that into a memory structure. As Chris puts it, "there is really very little difference between a player and an artificial intelligence character in Halo." This limitation creates more lifelike behavior for the AI characters, as they can be surprised, make mistakes and decisions based on their perceptions of what's going on around them. This is a key point in how the Halo AI works: Because the characters are forced to perceive the world around them, they are, in many ways, limited like the player by their senses - in their overall awareness of what is going on around them. "That all works through the same capabilities the player has," Chris explains.