I only played through it once, on a laptop, but I think this is why I liked Fear so much. There were times were something in my peripheral would move and I had to stop and take a look around and question if I really saw something move.

One of the more noticeable things that I remember was I had just climbed up a ladder onto a catwalk (I think) and started walking forward. I remembered that there was some ammo I had left behind that I should pick up and I turned around and right at the top of the ladder was the main antagonist standing there staring at me. I was surprised as hell and of course I jerked the mouse up and to the side, when I reset it back to face the guy, he was gone.

I don't know if something was supposed to make you turn around at that point and see him standing there for a second or not but it made me appreciate the game so much that I wasn't always sure about what was going on.