So I was asking earlier: What can horror do that's new and unexpected and genuinely scary in a market so saturated with the same tired plots and cheap gimmicks?
I was also thinking about what, exactly, makes us scared. What fears do we have? Are primal fears truly primal, or have we grown past them culturally? Is there some new fear?
I think at its core, fear is just anxiety, and anxiety is rooted, more than anything else, in the feeling of not knowing what's going to happen. Or, worse, knowing that something will happen, but not when, or where, or how. Everybody's familiar with anxiety. Of course, the greatest anxiety is death -- it could happen at any time, any where, and we could be taken entirely by surprise -- but other things produce that anxiety too. Humiliation, for one. Heartbreak and loneliness.
But where horror breaks off from other genres is in the way those anxieties are dealt with.
In order for horror to be really successful, you need to scare your audience. And in order to do that, you need to make them anxious about the plot -- which can only happen if they don't know what's going to happen (or do know, but don't want it to and are powerless to stop it -- which is so hard to do successfully, but so excellent when it's done well).
In real life, the scariest things are those that go unsolved or unexplained. If you never figure out the motive of the psycho-killer, if you never know for sure whether the horror is supernatural or all in your head, it's scarier. The most unsettling experiences you'll have are those that don't make sense. Once you can make sense of your experiences, you can start to feel at peace with them instead of puzzling over them again and again in your mind, working yourself up, showering yourself with anxiety.
But things that don't make sense don't make very good fiction.
Because in order to make a story really work in fiction, you do need to explain things, or at least offer some potential explanations that are actually satisfying. Contrary to popular belief, you can't make a good scary story out of just stringing together a bunch of unsettling images without context or plot. And viewers have a harder and harder time believing the motivation of "It's evil!" as sufficient as they become more genre-savvy. They demand better, and it's our job as writers to live up to those demands.
This is also probably why the scariest part of many horror movies is the first half, when all the threads are loose and everything can be strange and weird and inexplicable. Resolving those threads very rarely happens in any sort of satisfying way in popular horror fiction -- because it's hard, and because people don't necessarily demand that it happens.
So, the question: How do you leave enough loose ends to horrify your audience without just making your plot look lazy?