One of the great hallmarks of horror is the palpable build of tension. King did it masterfully in The Shining, with its slow-burn ratchet of terror. Matheson, when he wrote Hell House, created one of the single greatest nail-biters in literature. We, as writers, all aspire to create that sense of dread, that feeling that has our readers squirming in their seats, that moment when our readers, lost in our story, will leap out of their skin when a cat (or child or mouse) runs through the room. With movies, television, even radio plays, the listener has cues to build tension. Scary music, visual clues, or just a subtle drop in the actor's voice or a quarter-inch widening of the eyes can do it. But we as writers have no such tools on which to lean. We have our words and the page, and building any kind of emotional connection therein is difficult, to say the least.
Horror without tension isn't horror...It's merely a collection of horrific things. The emotional journey for the reader, without it, isn't there. So how can we create tension in our stories?
My own definition of tension as it relates to horror, and writing in general, is taking your characters to the edge and refusing to push them over. It's a subtle build of events, each one another notch on the ratchet, that brings him closer to the inevitable end. But more than just careening toward that end, it's in how he gets there that's important. It isn't enough to get your character from point A to point Z. It's the roadblocks (points B-Y) you set up in front of him that make the story. Each one brings him closer to the edge.
Take, for example, the classic ghost story. If the first thing the characters see is the horrible phantom in all its grisly glory, the story is basically over. We know what's coming and we're able to deal with it. But if it starts small, say with a single moving door, and builds, we see the terror mount. We see that ratchet tighten. We begin to feel that character's growing dread.
We can create tension, as author Gary Braunbeck says, through hesitation. A knock late at night. The character hesitates before opening the door. "Who could that be?" he thinks. Another knock. "It's so late," he thinks. Another knock. "I'm in a cabin with all my friends, and no one knows I'm here..." Another knock. The cliff drop is when he actually opens the door. The tension comes from his reluctance to open it.
You can also build tension mechanically in your stories, by using shorter phrases, single syllable words, things that bring the rhythm of your writing to a staccato fever like a pounding heart. You can build it with repitition, trigger phrases, or even with absence of description. Describe a puppy and leave out the word "puppy" and the fact that it's only eleven inches tall at the shoulder and what do you have? A fur-covered beast with needle-sharp teeth and claws. The moment you name it, your brain quantifies it, and can therefore deal with it. But if you leave it unnamed, leave the reader with the shadow under the door instead of what's making the shadow, the tension, the uncertainty, becomes significant.
Apply the above to any genre. In romance or erotica, keep the lovers apart though the reader knows they want to be together. In comedy, keep the punchline for as long as you can. Build the tension and string the reader along. It makes the payoff, punchline, or killshot much more engaging.