if youre making a singleplayer map, an awesome entity you can use for freaky effects is the brush-based 'trigger_look'. if you are in its volume looking at a certain thing for a certain amount of time, it will trigger. so you can use it to make things appear behind the player without him seeing or expecting it.
eg, in a scary map im making at the moment (inspired by this thread
) you walk into a small storage room, and if you stand in the half of it away from the door and look at the wall opposite the door, the door will close and lock
also, the only time ive ever been playing a game and been actually scared was on the Cradle map on Thief 3. go play it. now. totally fucking insane.
"Hey Scott. It's Dougal. Can I have my hair from the 1980's back?"