You obviously enjoy horror movies, as you've said many times in interviews.
Why haven't you written more horror fiction? -- Anonymous
That said, however, I do find that horror fiction is more difficult to write than I used to think. To make something scary with mere words on paper is not easy. There are too many variables the writer isn't in control of. The reader controls the speed, the pacing, the skipping of text, how their imagination perceives the imagery, etc.
This is different from films or audio experiences. In a film, all the viewer can do is watch and wait while the creator (storyteller) dictates the terms of the "communication." Verbal storytellers too have the advantage even though their main tool is simply the spoken word. Their listeners can't flip pages or put down the put mid story, etc.
One of the best tools I can use when writing horror fiction is that of "unease." If I can create an atmosphere that wigs outs the reader, then I can get in the first blow, so to speak, and keep the reader off his or her feet long enough to distract them from breaking the suspension of disbelief created by the story. After that, I've found that the trick is to step up the creepy, and not the gore, but only after a well-paced rest to let normality set it (even if it is a sort of heightened, eerie normality), thus stepping up the creepy by increments.
Or that's what I've found. As always, you're mileage may vary.