The Sydney Morning Herald has an article about Bungie AI programmer Damian Isla:

Animal emotions such as surprise, interest or disappointment can be useful parts of an AI creature. "They can have an impact in the game," he says. "Our characters have an incomplete knowledge model - they depend on line of sight. If you sneak up on them from behind they get surprised just like you or I do, because you are violating its expectations about where you are. It pays off in game play, because the character has a huge surprise animation and gives a yelp."

There's a particularly interesting bit on page two about the tug-of-war between AI programmers trying to make smarter opponents, and designers trying to make more fun opponents. A worthwhile read.