Horror is a film genre seeking to elicit a negative emotional reaction from viewers by playing on the audience's primal fears. Horror films often feature scenes that startle the viewer; the macabre and the supernatural are frequent themes. Thus they may overlap with the fantasy, supernatural, and thriller genres.Horror films often deal with the viewer's nightmares, hidden fears, revulsions and terror of the unknown. Plots within the horror genre often involve the intrusion of an evil force, event, or personage, commonly of supernatural origin, into the everyday world. Prevalent elements include ghosts, aliens, vampires, werewolves, demons, vicious animals, monsters, zombies, cannibals, and serial killers. Conversely, movies about the supernatural are not necessarily always horrific.