You have to find it in the moment, and that's one of the challenges of being an actor - especially a film actor - is that you have to maintain these heightened emotions for long periods of time. There's no trick to it. You just have to do.
People have always liked to be frightened. People love to feel that jolt of adrenaline. People love roller coasters. People love skydiving. These things that really get your heart pumping, and horror films are sort of a safe way to get that rush I guess.