It's actually a lot more sophisticated than that. Here's a great presentation from Valve about the "AI Director" in Left 4 Dead:<p><a href="http://www.valvesoftware.com/publications/2009/ai_systems_of_l4d_mike_booth.pdf" rel="nofollow">http://www.valvesoftware.com/publications/2009/ai_systems_of...</a>