Halloween is the ripe for horror movies, but what exactly constitutes a horror movie? Does it need a plot twist? Suspense? Mystery? Zombies? Blood and gore? Satanic influence? A presence of evil? Here is a psychological analysis of horror movies that can be used to write horror fiction as well.
Horror movies and literature must scare viewers and readers: That's a pretty basic requirement of the horror genre. But what does it take to scare people? Sensations of fear differ from person to person. Phobias (irrational fear) differ from realistic fear. Each person has a different level of credibility also; in other words different people are willing to believe as possible different things. Supernatural, aliens, monsters, zombies, vampires, demon possession, even evil and hell are concepts open to interpretation. And in order for someone to be genuinely frightened, he must believe that something is possible or exists. The viewer must also feel a sense of impending danger or personal threat.
Sensations of fear vs. sensations of being startled: Some movies rely on the element of suspense and surprise rather than attempting to produce sensations of fear. Loud noises, startling or graphic images, even blood and gore often produce a sense of being shocked or startled in viewers, rather than fear. Try watching a horror movie. If you find yourself jumping and your heart pounding, but feel no sensation of threat or danger, you are probably feeling startled, not frightened.
Sensations of disgust vs. sensations of fear: Gruesome movies are extremely bloody and graphic, but not necessarily frightening. There is nothing inherently fearful about seeing a human skeleton, flesh, blood, muscles or organs. Doctors see these things every day. Seeing a zombie dining on human flesh and feeling revulsion or disgust is not the same thing as fear. Again, a sense of threat must be present.
In summary, in order for a movie or book to produce sensations of genuine fear, it must communicate to the viewer a sense of personal danger or threat of danger. A real 'scary' movie must in fact scare the viewer, cause him discomfort, dread or a sense of impending doom. In order to produce that feeling, he must accept that the images shown in the movie are possible and believable.
Using these definitions, the most truly frightening movies are those which show the level of inhumanity and degradation that mankind is capable of wreaking on his fellow man. These horrors are the only the only kind that we can actually prove exist. Personally, I think some of the most frightening movies and books are the true stories of actual events. Helter Skelter, for example, scared me witless reading it at age 12. Because I remember hearing of Charles Manson and the very real atrocities he committed.

