How do you write a nightmare scene?

Suspense is the key element in writing a scary story.  This is done by having the reader know something the victim doesn't, and the secret is revealed one small step at a time.  For instance, in the famous shower scene in the movie Psycho, we see the woman taking a shower, not knowing the killer is in the room.  Then we see the killer's hand holding a knife and stabbing.  And the most chilling scene is the end, where we don't see the woman herself, but see bloody water going down the drain.  Let the reader's imagination fill in the gaps!!