I've been thinking about the issue of Whether or Not to Show the Reader the Monster. The story that I'm currently working on has a bit of a monster in it - or a strange creature, at least - so it has some personal weight. In any case, I am undecided about whether it's effective to describe a frightening sight when you're trying to scare a reader.
Mary Shelley and H.P. Lovecraft, two acknowledged giants of the genre, seem to take somewhat different approaches. In Frankenstein, Shelley does some initial description of Frankenstein's monster (black lips, yellow skin, etc.) but mostly she relies on the horror he inspires in others to convey his supernatural ugliness. She says: "No mortal could support the horror of that countenance. A mummy again endued with animation could not be so hideous as that wretch. I had gazed on him while unfinished; he was ugly then, but when those muscles and joints were rendered capable of motion, it became a thing such as even Dante could not have conceived" (Signet edition, p. 43). In other words, there's a lot of it's-too-horrible-for-words going on here.
In contrast to Shelley, Lovecraft (prone as he is to heaping abstract nouns on top of each other) is actually pretty specific about what Cthulhu looks like in "The Call of Cthulhu": squid head, dragon-body, yet somehow humanoid. He's also very specific about the slimy trail he leaves behind. It's all very Ghostbusters.
The problem is that I found neither Frankenstein's monster nor Cthulhu terribly frightening - at least not in visual terms. If I had to choose, I think I would come down on Shelley's side, because the horror you don't see is always more terrifying. In fact, the most chilling part of "The Call of Cthulhu" for me wasn't the actual emergence of the monster but the weird rituals of his cult and the vaguely referenced "strange disfigurements" of the people they stole for their practices. Yikes.
The tricky thing about horror, I think, is that it so quickly can become dated. "The Call of Cthulhu," for instance, reminded me strongly of Ghostbusters, but of course Ghostbusters only exists as it is because of Lovecraft. (Wibbly-wobbly, timey-wimey, as the Doctor would say.) To scare people, you must constantly be finding something new or surprising. In actual fact, a book hasn't scared me properly since Bunnicula gave me nightmares. The closest I've come to being scared has come from particularly shadowy psychological moments in Harry Potter (like the explanation of the Unforgivable Curses; or the conjuring of the Dark Mark; or the exploration of Voldy's early life). Then there are, of course, some of the lovely creepy characters on Doctor Who, particularly Steven Moffatt's creations. Unfortunately, I don't think the chills Moffatt creates are available to me as a writer, because they rely on the visual shock of, say, a gas mask or a twisted clockwork harlequin.
What gives me chills now are brutalities such as you see in the proliferating crime shows like Law and Order, films about serial killers, and even the latest BBC adaptations of Sherlock Holmes. And frankly, I'm not at all interested in emulating those chills in my fiction. They are not the fun and thought-provoking chills of classic horror: as far as I'm concerned, the degree to which they rely on the victimization of women is incredibly disturbing and says something about the sadomasochistic impulses of our current culture. But that's another story.