As we hit the halfway mark, let's do something different. The next five non-specific ideas follow a common theme: Things that are usually present in horror movies, but really don't need to be (or at least, it would be interesting to see what happened if they weren't).
48. Less music = more scary.
The establishing shot of the city where the story takes place does not need to be accompanied by ominous creepy-little-kid-choir music. Save the atmospheric music for matching atmospheric visuals. Similarly, if scaaary string music is slowly building up to a climax as the helpless blonde chick is exploring the killer's house alone, you automatically know that one of three things will happen when the music reaches the height of its tension: A) The killer will jump out; B) A cat will jump out; C) A cat will jump out and then a second later the killer will jump out. Just once, I'd like to see a character going about their regular business, with no accompanying musical warning, and then the killer jumps out and rips their face off. No warning, it just happens. That's fucking scary.
49. Fear should stand out.
There is no need to make everything disturbing. If the script calls for someone to eat a hamburger, it doesn't need to be raw and bloody and run down a guy's face just because it's a horror movie. People don't need to trip and fall every time they run, but if someone does happen to run into a piece of furniture, they don't always need to get a head injury or lose a limb. Sometimes people just get minor bruises. Even in a horror movie, not every building needs to be creepy in every possible way. There don't need to be glassy-eyed severed animal heads lining the walls of every home. Not every light needs to flicker. The power doesn't need to go out every other day.
50. Not everything makes noise.
If movies were reality, every time someone was startled, a loud, high pitched squeal or orchestra hit would suddenly come out of thin air. In reality, the killer would probably freeze, look around, and say "what the hell was that?" That isn't very scary. I think it's equally ineffective in movies. If the only way to scare the audience is with a sudden loud noise, what's on screen probably isn't all that scary.
Also, people do not make swishing noises when they move. Neither do cameras. So why would there be swishes and screeches every time the point of view changes? Drawing attention to the camera is a sure-fire way to ruin immersion in a genre where immersion is key. Making it seem like the camera is in a wind tunnel, or being dragged along a steel floor, ruins immersion. I'm looking at you, Saw movies.
51. Flashes of random shit are stupid.
Remember that part in The Exorcist when the pale face of evil suddenly flashes on the screen so quickly that you can barely see it? That was scary. It's not scary when every horror movie since then has decided to insert random pictures and "frightening" faces between every scene transition. It's even dumber when accompanied by a high pitched squeal (see above). Do the characters get a case of titinus and hallucinate every time they enter a new situation? If not, stop showing me cheesy irrelevant crap.
52. Special effects are not scary.
I recently watched a zombie movie in which the first zombie appearing on screen started shaking in fast-motion and going all blurry, due to some camera effect. What? Did the zombie actually do that? Or is that representing the main character being confused and scared? Because you know, when I'm scared, the world doesn't go into time-lapse photography, and I don't see people doing things they're not actually doing. Distortions of reality can be fine when done right (the end of the Dawn of the Dead remake used shaky-cam to good effect, like a confusing nightmare), but only if there's a good reason for it. "Ooo, we got this new computer program that makes everything look blurry and shaky! That's a scary effect, right?" is not a good reason.
53. Know the rules, but don't always play by them.
A good horror movie is like a good piece of music. It follows the rules of composition that you expect, it sets up some of its own rules, but then, when the moment is right, it blows your mind by shattering expectations.
I think it was pretty brilliant when Drew Barrymore died right at the beginning of Scream. Expectations were set up that she would be the main character of the film, both in the movie and outside of it (e.g., being prominent on the poster, being the most recognizable actor), and then those expectations were obliterated.
At the same time, like good music, pace is everything. If a movie blows its load right at the start, it's gonna be flaccid for a while afterward. A slow build, getting more and more terrifying with every scene, is effective. Setting up expectations of a slow build, then having the terror jump out before its scheduled appearance, is even more effective.
See also: 100 Original Ideas for Horror Films, #43 to #47.
Note: This post was in the works for a while, but I was inspired to finally publish it by a dude named Eli, who emailed me asking for some input about horror movies because he is interested in shooting an independent horror film. I hope my opinions here are useful in some way, and good luck with the project.