|Basking sharks are actually harmless. Not that they look it.|
I've already mentioned this story, in my post about too much imagination. It bears repeating, though, because now I'm going to talk about how to tap into that to write something that scares people.
There are various types of horror, but the two I'll discuss here are gross-out and frightening horror. I'm sure they have better names elsewhere, but that's what I call them, and this is my blog.
Gross-out horror is the type where your stomach churns, and you swallow back the urge to vomit. Slimy monsters, scads of blood, viscera and popped eyeballs, all invoke gross-out horror. The most effective way to gross out your reader is to make them experience the horror with a sense they weren't expecting. The closest I came to throwing up while listening to an audio book was at the description of a beheaded man's blood making a whistling sound as it jetted out. I've heard plenty of descriptions of the sight of blood, about how it sprays everywhere, but that whistling sound sidesteps cliché and makes it something I can imagine.
Frightening horror, on the other hand, is horror that makes your heart beat faster, your pupils dilate, your palms sweat. You relate to the characters' fear, and you feel it for yourself. Frightening horror will stick with you long after the book is closed or the credits roll. The next time you have to cross a room after you've shut out the lights and you see a dark shape in the corner, it'll leap to mind.
Or, maybe you're much braver than I am, and you'll only get a temporary adrenaline rush while you're reading or watching that scary movie. If so, I envy you.
In any case, I've always thought gross-out horror is easier to write than the frightening kind. It's one thing to be over-the-top gross, but quite another to draw the reader into your imagination, and make him or her feel what you feel. I know a lot of horror writing uses both gross-out and the frightening kind of horror, but I have a lot more respect if something can make people feel.
As I said above, the best way to use gross-out horror is to put the reader in the moment by invoking unexpected senses. But how does one scare people?
Generally, it involves a sense of anticipation. Once you show the scary monster, the reader (or viewer) can see its weak points. They can see that it works by the same rules as everything else. It's much scarier in their imagination. I know people who, upon seeing the whole creature at last in Alien, laughed, because it wasn't as scary as they'd thought from the random glimpses throughout the movie.
It also involves going after what really scares people. At our core, we all fear death, we fear pain, we fear rejection, we fear what we don't understand, and we fear being alone. The more of those you can invoke without naming them, the more you're going to frighten your reader.
You don't need to be a fraidy cat like me to figure out what will scare people. It helps, but it's not necessary. If you've ever had the sort of dream where you've woken drenched in sweat, certain that the creeping horror from your dream is under your bed and will grab your ankles if you get up, you know fear. Use that.
Do not, however, transcribe your dreams. My most frightening dreams are about the most mundane things, and I often laugh about how scared I was when I tell other people.
For an excellent example of how to write horror, I would highly recommend this post by Cherie Priest (link goes to her blog), which scared me so badly I had to get the cat to accompany me to the bathroom on the sunniest afternoon in May history.