Presenting the illusion of a living world is always a challenge for a game developer. Real people often have quirks that even advanced artificial intelligence has trouble mimicking. Some developers compensate for this by giving the AI more control, but this too has flaws as the AI can become unpredictable. For S.T.A.L.K.E.R., the developers at GSC decided to combine the two approaches and get the best of both worlds.
Driving the behavior of the game's hundreds of NPC characters is the artificial life, or A-Life, system. More than a year-and-a-half was spent on the A-Life system alone but the results are impressive. Characters mill about in the game, but aren't restricted to any one area. Other stalkers may take the same missions you do and end up competing with you for the prize. Beasts might be native to one area, but once released have the potential to show up anywhere.