I Know What You've Watched Last Summer...

A little tip that I've stumbled upon once, and since then I'm using quite a lot: A great way to find what type of horror game it will be, is by getting to know what horrors movies the players really like or found frightening. If the players, for example, have found that The Birds and Jaws are truly terrifying, going for a horror game about nature goes mad might be the right type of game to GM. If the players, on the other hand, have been frightened by The Shining, maybe you should throw the slasher game that you've prepared and go to the psychological realm.

It can also be used, of course, to avoid failures. If the players start to laugh every time that they watch a slasher, maybe slasher is not the right type for them. If they find body horror as too horrifying for their taste, maybe there's another direction that you should seek instead of this one.

There's one danger, with using this technique: If you GM a Gothic horror game for your players, and they know the genre well enough, they might get less terrified as they know the common tropes. Here comes to the rescue the other part of this thing: After you know what the basic feel of the game should be, add a little color from other types of sub-genres. If the players found Gothic horror as the most frightening genre, adding a little bit of nature goes mad and psychological horror will surely confuse the players a little bit, just enough to get to the dreading parts...