I often write on this blog about what makes a good scary story. However, it occurs to me that part of the problem the genre faces is that horror, at least, that brand of horror which I prefer, derives from the unknown and the unexpected. And when you take in a work that you know and expect to be frightening, you will expect to be frightened, which makes actually being frightened less likely.
One way around this, of course, is to disguise the horror. Place it within the context of another sort of story. This mixing of genres is quite common nowadays, and that’s a good thing for this purpose. As I’ve said in the past, two of scariest things I’ve seen in video games are Ravenholm in Half-Life 2 and the cemetery in Jade Empire, neither of which are actually “horror” games.
Another way is to begin the story by using familiar tropes of the horror genre, so that the audience expects a conventional horror story, and then switching things up somehow so that the frightening part comes from a wholly unexpected direction. But this is easy to say, and very hard to accomplish.
The last solution I can see is to take the audience expectation of horror and run with it. By that I mean use the sense of inevitability about it to your advantage in heightening the horror. This might be why the film The Omen works so well. The scary thing about it is that the events of the film have already been prophesied, and sense of inescapable doom weighs upon everything.
(This film also does a good job messing with the viewer’s expectations. For instance, there is one character whose death is foretold early on in the film. During the film’s second half, this character is several times put in obviously contrived, dangerous situations–e.g. wandering alone into deserted catacombs–and yet he survives them, defying audience expectations.)