First, and sadly, that particular genre is frequently depicted as bloody, violent, gory and disgusting beyond words. Many people avoid reading in that genre just because of that. And, while it's true there are some written that are filled with mindless violence, little plot, lots of blood and shock, there are many others that are just oozing with atmosphere, emotion and a great story.
Writing for horror means the writer has to be skilled in many ways as any other writer, developing characters, creating plot twists and snappy dialog in order to create a gripping tale. And the horror writer goes a step further, taking us by the hand to lead us into the dark unknown. I say dark because that's usually the kind of unknown the horror writer is exploring.
I enjoy reading some horror which is why I'm contemplating writing it. I enjoy the chills and the anticipation, the twists and the unexpected. Be they ghosts, creatures, slithering things, aliens intent on mayhem, or the great beyond.
So how does a writer get there? Well, as with all writing we know the reader is willing to suspend disbelief, to enter the world of your creation, but the writer has to give the reader a reason to read. There has to be a beginning hook of some kind. The story can begin normally and slowly draw the reader into 'weirdville' or it can start out strange and just accelerate.
Description is exceptionally important while writing horror. It's a great place to practice if you want to send shivers up your readers' spines. I've noticed short passages, written well are the key. Enough to really get me on edge, then draw back a bit. The writer who's careful not to overdo it, to hit just the right balance is the one who creates the vivid details by tapping into all the senses and alternately picking those most appropriate to the scene.
The writer, no matter what kind of characters he or she creates, be they vampires, werewolves, ghosts or aliens, must create characteristics within them that make them appealing and yes, no matter what, human. The characters don't always (horror? - maybe hardly ever) have to be likeable, but they do have to be engaging. The reader must have characters that can be identified with.
Check out some of the more recognizable horror authors: Dean Koontz, Robert McCammon, Stephen King, Anne Rice. Notice the charisma they've injected into their characters via appropriate names, appearance, action. Notice how they've created characters who aren't perfect. The hero or heroine of the tale often screws up, thinks unkind thoughts or does something 'politically incorrect'. If the reader can empathize with the character then the character takes on life and breathes.
I've also noted that frequently the main character in a novel has the least amount of description. This trick allows the reader to imagine themselves in the same situation - no matter their own 'real world' attributes. So keeping it simple seems the best route.
Now I'll mention this because it's so true, for horror or any writing for that matter. Don't name your characters too close together. Don't write a story with a "Mary" and a "Marie", a "Mat" and a "Cat", a "Harry" and a "Mary". Anything which is just to close confuses the reader and a confused reader is not a happy reader.
Hmm, after writing this I may be writing a horror tale soon. If you're so inclined as well, have at it, Read, a lot, then create your own thing that screams in the dark.