A horror film aims to frighten the audience and elicit feelings of fear. To be successful, it must contain at least one frightening element that triggers fear, such as the unknown, terror, torture or surrealism. If viewers feel scared by the film's content, plot or characters, it is more likely to succeed financially. Common techniques used in horror films to promote fear include dark lighting, eerie music, slow pacing and vulnerable protagonists. There are also various horror genres that overlap, including supernatural, psychological thrillers and science fiction horrors.