Lately I've been writing about action films, and it occurs to me there is much overlap between that genre and horror. Both are centrally concerned with the struggle between more or less normal characters and some kind of destructive nemesis. Both usually involve violence and fear.
The main difference is in the primary purpose of each genre--what it is principally trying to do. The horror film's primary purpose is the presentation of a deformed destructive being (DDB), ie, the monster. The action film's primary purpose is the presentation of a physical fight between a hero and a villain. The horror film mainly wants to show you the monster; the action film wants to show you a fight. This difference is marked between two closely connected films, Alien (1979) and Aliens (1986).
Alien is the story of a grotesque extraterrestrial that gets aboard a spaceship and starts killing the crew one by one. Though the DDB is rarely shown in full detail, the focus of the movie is this monster, and the prevailing feeling is one of fear. Attempts to fight the alien with makeshift weapons are mostly ineffectual. At the end, the final girl, Ripley (Sigourney Weaver), barely manages to defeat the alien by having it sucked into space. Alien is clearly a horror movie: it is all about the DDB.
Aliens, the first and best sequel to Alien, is also about the species of extraterrestrials that caused such havoc in the first film. Weaver returns as Ripley to contront the creatures again. But this time she is accompanied by a squad of space marines wielding heavy weaponry. In a series of dynamite action scenes on a desolate planet, she and the marines wage battle against hordes of aliens, only to find themselves outfought. The few survivors devise a plan to leave. With minutes ticking before the area is about to undergo nuclear explosion, Ripley has to stay and fight more aliens in single combat--including the dreaded alien queen--to protect a little girl, Newt. Ripley and Newt escape on their spaceship, only for Ripley to have to face the alien queen again. Ripley does so in a mechanical loader that resembles yellow battle armor. After exchanging punches, Ripley again forces the alien to be sucked into space.
The difference is tremendous. Aliens is not a horror movie at all--it is an action movie. The point is not primarily to show the DDB but to show the hero (Ripley and her marines) having one physical fight after another with the villain (the alien queen and her aliens). Ripley's heroism is emphasized by giving her a child to protect; the alien queen's villainy is emphasized by having her focus on attacking the child. Alien and Aliens show how closely the horror movie and the action movie are linked--so closely that one can be transformed into another with the addition of a few more weapons and some space marines.
Deformed and Destructive Beings: The Purpose of Horror Films