Last edited Tue Jan 26, 2021, 12:37 PM - Edit history (4)
There is absolutely an accepted structure for screenplays, first identified and written about by Syd Field. Many new screenwriters don't like this idea of there being rules (I didn't), but nearly every successful film obeys them to a large extent.
No doubt, working on the protagonists is much more gratifying than the antagonists. If you ask ten screenwriters their opinions on development of antagonists, you will get ten different answers. There was a time when tremendous emphasis was placed on development of the antagonist ("the more interesting the antagonist, the better the film" ). This is my personal opinion: It depends on whether your antagonist is truly a "villain" (think of the serial killer your detective is trying to capture) or an authentically-interesting and important character that triggers change in your protagonist (a movie like "As Good As It Gets", or deeply psychological movies like Lars von Trier's works like "Melancholia" or "Antichrist" ). I personally don't believe in overdeveloping unlikeable antagonists; it's a personal preference. For example, I don't care about the backstory of the villain in a James Bond or Alfred Hitchcock movie. But in dramas, it becomes much more important to fully develop antagonists, including writing up their backstories even if you never refer to their histories in the finished screenplay. Think of a fascinating antagonist like Nurse Ratched in "One Flew Over the Cuckoo's Nest" or Fredo in "Godfather II" (or Colonel Kurtz in "Apocalypse Now" or Faye Dunaway's character in "Chinatown" ).
For everything I'm typing, there are many exceptions. Protagonists don't have to be likeable, but they should be interesting and complex. ("Taxi Driver" ).
To make a really long story short:
A screenplay has three acts. The first act ("The Set-Up" ) is approximately 28 to 33 or so pages. The second act ("The Confrontation" ) is approximately 60 pages. The third act ("The Resolution" ) is usually 20 to 30 pages. When all is said and done, a page approximately equals one minute of screen time, but this varies on a page by page basis based on the amount of dialogue vs. description.
A screenplay is basically a series of sequences broken down into scenes, logically and properly ordered to escalate interest, leading towards a change or epiphany in your protagonist (unless your story is about a protagonist "who refuses to change" ).
Each scene has some level of conflict. It can be extremely subtle ("The Big Chill", "Grand Canyon", "Girl Interrupted" ) or it can be people screaming at each other. Some tension must be present early on, and then sustained.
Sequences end with "reversals", which toss the story in an unexpected or interesting direction.
Major plot points occur at the close of Acts I and II.
Something relatively important should occur around the midpoint (approximately page 60). The joke used to be that if your screenplay contains a romantic relationship, page 60 is when they finally sleep together. 
It is the collision of the protagonist with the antagonist that creates the change, unless you are writing an ensemble film or a film in which "society" is the antagonist.
You may want to invest in screenwriting software.
You have three to ten pages to hook your reader.
Expect to make mistakes. Completely outline your entire story before you start writing. Never begin a screenplay without knowing what the third act will be or the last few sequences.
You can break all these rules once you've learned them and made them work!
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