The movie script is the backbone of every feature film. It consists of dialogue, characters, locations, props and story. It is used by the crew during every phase of production. Movies are green-lit, people are cast based on the fundamentals of the characters and sets are built based on the strength of the script. The script is used again in post-production to edit the many hours of random footage into a compelling narrative.
The recommended length of a screenplay is about 120 pages. It does not have to be exactly 120 pages, but the closer to that number, the better. A general rule of thumb is that one page of a screenplay equals one minute of screen time, which would make the ideal screenplay play at two hours. Obviously, if you are established within the industry and have a celebrated career you can turn in a screenplay that is as long as you like. James Cameron, writer and director of "Titanic" and "Terminator 2: Judgment Day", can turn in a 400-page screenplay because he has already made some of the most successful movies of all time. A new writer, however, will have his screenplay thrown out if it is too over or under 120 pages.
Screenplay format is very specific and is an art unto itself. Stage directions are written as if they were normal paragraphs, but they don't describe anything that can't be physically seen. Character thoughts have no business in the stage direction. The first time a character appears in a scene, his or her name is capitalized. This makes it easy to identify which characters are in which scenes when skimming through. Character dialogue is centered in the middle of the page. It is recommended before writing a screenplay that you read as many as possible to get a feel for the format. Many Hollywood screenplays have been published in book form and are available at local bookstores and via the Internet.
Within the average 120 pages of a script are three acts, each running roughly 30-40 pages. These map the emotional arc of the movie. The first act introduces the characters and the conflict. The second act has the protagonist running into trouble and resolving that conflict. The third act is the climax and resolution of the story. It is also important is start the script with a bang. Script readers (who are people hired to read scripts and write reports on them for executives who don't have time to do it themselves) are instructed to only read the first ten pages unless the script gives them a reason to continue. If a script doesn't start out with a proverbial bang, it will get thrown out.
A common misconception from beginning writers is that script format doesn't matter. "Who cares what format it's in?" they say. "I've just written brilliance!" The reality couldn't be farther from that. Executives receive dozens to hundreds of scripts a day. They don't physically have time to read them all or even pay someone to do so. So they are literally looking for any excuse to put one down and pick another up. If your screenplay isn't properly formatted, it's an excellent way to get them to throw yours in the garbage.