Valve Video Interview on Half-Life and AI
Marc Laidlaw, script and story writer for Valve has recorded a video interview on the AI used in Half-Life. According to Laidlaw, the AI developed for Half-Life is advanced enough that NPC reactions will be different each time you play the game. Valve has committed itself to providing single-player games with dynamic AI to keep each play through fresh and extend the shelf life of your game.
Take a look at the gameplay footage provided to get an idea of the variances the AI interaction can have.