In Three Acts
Most screenplays are built around the framework of three acts. Act I is generally a quarter of the script, Act II is usually half, and Act III is the final quarter. So for a 120-page script, the first thirty or so pages will comprise Act I, the next sixty Act II, and the last thirty Act III.
The most important part of the screenplay is the beginning — the first ten pages of Act I. They must dazzle. They must immediately grab the reader's (and later the viewer's) attention. They must also do a lot of legwork: introduce the main character, establish the story's setting, let readers know the type of movie they'll be watching (comedy, thriller, science fiction, western), and get the story going. If you've seen the beautifully photographed film Out of Africa, you'll remember that the main character begins the story with the line “I had a farm in Africa,” and viewers immediately see, as the setting shifts from a woman sitting at a writing table to a hunting scene from her youth, that the story will be a remembrance, and from the clothes and the lush scenery and the music and the introduction of a flirting male character that this will be a story of relationships in a distant time and place.
Each page of a screenplay is equal to approximately one minute of screen time. That means a standard-length script of 120 pages will become a movie that's about two hours long.
Approximately twenty to thirty pages into most scripts, a plot point is introduced. This is an event that occurs within the story that sends its characters off in a new direction. In Out of Africa, the female lead, who hasn't married and is feeling condemned to a small life in the place where she was born, arranges a marriage to a friend, and the two leave their homes in Denmark to start a life together in Africa.
Following the first plot point, Act II turns up the tension. During Act II, the characters confront and deal with many issues and the conflict increases. In Out of Africa, the lead characters fall in love, war causes change, a business fails, and the lovers find that they can't live happily together.
Approximately three-quarters of the way through most scripts, the major issues come to a head in a second plot point — the climax, where another unexpected shift takes place. In Out of Africa, after the main characters sadly decide to go their separate ways, the male lead is killed in a plane crash.
For the last quarter of the script, in Act III, the denouement, or resolution, takes place. In these pages, any remaining issues are worked out and wrapped up. In Out of Africa, the female lead returns to Denmark and begins to write about her experiences, bringing the story full circle.