Decide the length of your proposed screenplay. This will determine the amount of writing that you will need to accomplish each day to reach your goal by the end of 3 weeks. For the purposes of this article, the assumption is made that you have already come up with an idea, decided on a genre for it, identified who your target audience is (i.e., chick flick, teenagers, families, etc.), and have a working knowledge of what a screenplay looks like. Since one page of a correctly formatted script is equal to 1 minute of screen time, let's say that you want your movie to be 90 minutes long. This means that you will have to write 90 pages of screenplay.
Create a working outline for yourself on Day 1. Since screenwriting adheres to a three-act structure of conflict, complication and conclusion, the easiest way to do this is to divide your total number of pages (in this case, the number is 90) by 3. This means that you will have 30 pages to devote to each act of your movie. Take an 8 1/2-inch by 11-inch piece of paper, lay it out horizontally and fold it into three equal columns. At the top of the first column, write Act 1; at the top of the second column, write Act 2; at the top of the third column, write Act 3. Under each act, briefly make a list of what is going to happen in each of these 30-minute increments. The objective of this exercise it to make sure that you not only have an equitable distribution of events but that each act also escalates the stakes for your protagonist and forces him to take bigger and bigger risks.
Write the first 10 pages of your script on days 2 and 3. The first 10 pages (first 10 minutes of your movie) are critical, because you need to hit the ground running and hook your audience into wanting to stay for the rest of the movie. In the first 10 minutes, you need to introduce your protagonist, establish the core conflict or quest that will drive the entire film and communicate the genre and tone of the film to your audience (i.e., comedic, scary, magical). Within these first 10 pages, you should also embed any foreshadowing that will later prove to be useful late in the third act. Example: In "Back to the Future", we learn early in the movie the exact date and time the old clock tower was struck by lightning.
Write pages 11 to 30 of your script during days 4 to 7. This constitutes the first act of your movie and should not only introduce the villain and/or obstacles that are going to thwart your hero's quest but also deliver us to the first major turning point or cliffhanger. For instance, suppose your hero is in danger of losing the family farm to the evil banker. As tight as things are for him financially, at least he knows he can always go to his favorite aunt and ask for a loan if he really gets desperate. What happens, though, if his aunt gets hit by a bus on page 26 and dies 4 days later? The hero is, thus, forced to decide whether he's going to give up or aggressively seek out a new way to raise money by the bank's impending deadline.
Write pages 31 to 60 of your script during days 8 to 14. During Act 2, things go from bad to worse for your protagonist. Not only is he dealing with the panic of raising the money to save the farm and the grief of burying his aunt, but he is now also in danger of losing his job, losing his girlfriend and finding out that he has a cousin who is going to challenge him for his share of the aunt's estate. By the end of the second act, the cousin approaches him with a proposal; he will loan him some money so the hero can make his rent payment and buy foo, but--as a show of good faith--he wants to be added to the deed on the farm.
Write pages 61 to 90 during days 15 to 20. You'll notice that you have 1 day less than you've had for the first two acts, but by now you have probably built up a good head of steam and are anxious to tie up all the loose ends. (If you're a particularly prolific writer, you may even have bought yourself some extra time by writing your first two acts in less time than was allocated.) Act 3, however, is not smooth sailing for your lead character. The reason is that he didn't completely trust his cousin and turned down the offer. He is now being threatened with eviction from his apartment, has had to rely on the kindness of friends to feed him meals and is being pressured by the banker that the deadline is only a few days away. All seems lost. But wait! What if it was foreshadowed in the opening pages that he bought a lottery ticket? (All right, it's a tad contrived, but you get the picture.) What if the homeless person he bought a meal for in Act 1 turns out to be a millionaire in disguise who was looking to see if there were any kind people left in a heartless city? What if he discovers that the alleged cousin is actually an actor whom the banker hired to help him commit fraud? Your third act should not only resolve the conflict in a way that surprises the audience but should also allow the hero to have experienced a character arc that enabled him to grow and embrace a new awareness about himself and about life.
Read your entire screenplay on Day 21. Better yet, recruit friends to read all of your dialogue out loud (to test whether it sounds natural) and to provide you with constructive feedback.
Even if you find yourself going back and rewriting certain scenes, tweaking with the dialogue, or dropping extraneous characters, you've still managed to complete a screenplay in 3 weeks. Psychologists have shown that 21 days is the amount of time required to either embrace a new habit or break an existing one. If you have previously procrastinated about starting or finishing a project (a screenplay being a great example), your determination to stick to the 21-day schedule in this article is probably going to result in your being so excited about the process that on Day 22 you won't be able to stop yourself from wanting to write some more.