No record exists of the Stalker preview I first read in a copy of PC Gamer magazine over 20 years ago. So I'll tell it to you like a tall tale from the exclusion zone, shared over a bottle of vodka. They say there's a shooter where no mission plays the same way twice. Where, on the first try, you might battle a gang of bandits in an old factory for ownership of an artifact. And on the second, you might arrive to find those same bandits already dead, mauled to death by a pack of mutated dogs.
The stories of Stalker: Shadow of Chornobyl's earliest builds all have this unpredictable quality. They describe an 'A-life' system which enabled NPC stalkers to pick up jobs from dealers and set out into the Zone on their own steam, trading with neutral parties and fighting the enemies they encountered. This activity occurred regardless of where the player was, or what they were doing. In fact, in 2004, GSC Game World was telling journalists that AI stalkers could actually beat you to the best of the game's eight endings, uncovering the mystery of the Zone for themselves.
