Screenplay Versus Novel
Like the novel, a screenplay is based on theme, characters, plot, setting, and dialogue. It also requires gripping conflict and pacing that doesn't lie down on the job.
A conflict is made up of a series of crises, straining points in the conflict that cause a realignment of forces or some change in the character. The plot is created by the selection and ordering of these crises. In each scene's internal crisis, an action is taken or not taken, or a decision is made or not made knowingly by the characters.
— Irwin R. Blacker, screenwriter, teacher, novelist, and television documentary writer
The format of a screenplay is completely different from that of a novel. It is structured in three acts and generally runs between 110 and 120 pages; that's because scripts fewer than 100 pages or over 120 are less likely to be bought and produced due to economic reasons. Shorter films, which run closer to an hour and a half than two hours, are sometimes shunned by moviegoers who think they won't get their money's worth. Longer scripts can require an enormous budget to film, making them less attractive to production studios, no matter how good the story. If you hope to have your script end up being shown at the Cineplex, you'd be well advised to keep it to the feature-length standard of between 100 and 120 pages.
Did you know that one of the reasons most movies run two hours or less is so that they can be shown in theaters many times a day? Long movies can't be shown as often, lowering the theater's revenues.
Another big difference between the novel and the screenplay is that the author provides a line of information at the beginning of each scene. This information, called the slug line, tells the reader about the look, time, and setup of the scene. Lines of description about what's happening in each scene are also provided, making a completed screenplay a blueprint for filming a movie as well as a vehicle for telling a story.