One of the biggest elements of horror films is the shock factor. As an actor, you can learn to use this convention to your advantage. In most horror films, there is a scene that seems relatively benign, or a scene that begins calmly. Don't ignore the importance of this peacefulness. If anything, you should play the calmness to its fullest potential. The calmer you are in the scene, the bigger the pay-off will be when the shocking action occurs.