It seems to me that horror, as I'm trying to write it, actually encompasses everything I want to write. But on the other hand, if a theme comes along and takes the book in a different direction that turns out not to be horror, then that's fine. Horror fiction, particularly supernatural horror fiction, came out of the mainstream. There's hardly a major writer of short fiction who hasn't written a ghost story at some stage, and often that may be what they are mostly remembered for…What has happened is that books have been packaged by publishers into genres and it is this which has caused the split between mainstream and horror fiction. Obviously there is some fiction which is pure horror, and there's nothing wrong with a story that sets out to do nothing but frighten the reader any more than there's nothing wrong with a comedy which sets out to be nothing but funny or a romance that sets out to do nothing but make you take out your box of tissue. At the same time, I think that horror fiction is often much more than that, and that's certainly the kind I've always tried to write.
Ramsey Campbell in How to Write Tales of Horror, Fantasy & Science Fiction, edited by J. N. Williamson, 1991