Most film scripts involve three acts. The first introduces the story, characters, and conflict. We usually get a sense of the protagonist's "ordinary world" or their normal routine (see Joseph Campbell's fantastic book The Hero's Journey). This act always have some kind of hook to make the audience want to keep watching. Horror movies tend to open with a monster killing someone or some kind of frightening murder. The end of act one is typically the point of no return. Everything has been irrevocably changed by some event or choice. In Aliens, this is when Ripley decides to go with the colonial marines to investigate LV-426. The second act deals with the main conflict and typically ends with a dramatic reversal. Act three builds to a climax and then resolves the story. Scripts are anywhere from 90 to 120 pages although of course, they can be longer or shorter (Steven Soderbergh's script for Solaris was just 70 pages). Each page equals about one minute onscreen.
