We know right away that it's incredibly important to take STALKER at your own pace and playing-style. A game this large and open-ended is all about the choices you make.
You'll begin the game in a village populated by mutated, high-tech poachers, known as STALKERs. From here you can explore the area, admire the "A-Life" system (as STALKER's AI is known) in action and pick up other missions. The missions received here form loose plot points designed for you to discover your origins (amnesia is a theme). But these breadcrumbs of plot are merely suggestions.
Unlike most FPS games, there's an ecology with the AI that governs STALKER. At one point, we were directed to a small group of STALKERs whom we would be able to trade with, but were shocked to find only their bodies, some bloody smears and a few sated boars. But in another player's version of the game, they were alive and friendly. Truly, each gameplay experience will vary.