For me, part of the answer involves facing our fears voluntarily.  A horror story (and particularly a live performance) can bring emotions home in a very personal way, giving us the opportunity to reflect on our own responses to situations that – hopefully – we’ll never encounter in our daily lives.

Another part of the answer, one that I’m still trying to understand more fully, is the connection between fear and awe, Burke’s terror as the path to the sublime.

There’s another aspect to horror that I also believe is important, an excellent description of which I found in the gaming book Heroes of Horror:

Horror lavishes attention on the dark side of fantasy—painting horrible monsters in bold strokes but also including plenty of subtle shades of gray, particularly in the complex motivations of the heroes. The intent is not to glorify evil but to cast good in sharper relief, even if good is represented as a tiny candle flickering weakly in a darkness that threatens to overwhelm it.

Fictional horror allows us to consider all of these things in a clearer light because it disallows one of the strongest and most appropriate reactions to real-life horror: “How do we stop this from happening?”  In a horror story, we know we can’t stop the inevitable; we can only gaze into the dark mirror and learn what we can from the reflection that stares back.

The distinctions between horror and terror have been explored, particularly in the realm of gothic literature, but I’d like to suggest a simple definition:

A situation is terrifying when (a) it provokes a fight-or-flight response, and (b) neither fight nor flight is possible.

I like this definition because it covers not only the gothic terror of unknown impending doom, but also the melodramatic strapping of people to logs in sawmills (which would seem a lot more terrifying and a lot less cliché in the first person).  It’s a useful framework for building a scene.  It also strikes me as a decent starting point for developing musical themes.