As adrenaline hits your brain, your amygdala -- a small almond-sized portion of the brain -- starts to override your rational reactions, Amunatequi explained. As a result, a frightened person will simply react to a scary situation rather than sit down and rationalize their reaction, for example, to a killer clown at a haunted house. The response would have helped keep our ancestors alive if they encountered any danger by helping them put all their energy in into fighting or fleeing the danger, Amunatequi said. "If you [survived] the encounter, you have to dispense with all that riled up energy you have, you end up really jittery," Amunatequi said. "It's designed to keep you alive. You’re going to cash all these chips in to get away," from the predator.
You may think that a fake scare doesn't mean the same response, but Dr. Frank Farley, professor at Temple University and former president of the American Psychological Association, said the reason a horror movie can strike terror is because it relies on your brain making connections between fake and real events.