Structure is one of the most important elements in screenwriting, and it’s also one that a good writer could spend an entire screenwriting book explaining. Having read several of those books, I can attest that everyone has their own method of explaining the three-act structure. Different books and screenwriting professors might have different terms for certain structural details, but in the end most scripts can be broken down in one way or another. For a very detailed breakdown, this reader suggests the one that Blake Snyder discusses in his book Save the Cat.
However, in most cases, I usually discuss structure in a less specific fashion when I do coverage. Here’s the short, most basic breakdown I tend to work from:
Typically you want the inciting incident to happen in the first fifteen minutes of the movie. This is the moment that puts the main arc of the story into motion and often ends up defining the script. In most cases, you’ll find it close to p. 12-15. Then, at the end of Act One, there will be a major turning point in the plot that sends the story in a new direction. This usually happens in the range of p. 25-30. First acts generally conform to this pattern, whether the second act is 30 pages or 60 pages. If your main story hasn’t gotten some advancement by p. 30, it’s usually time to start tightening the pace.
Act Two has three turning points, and usually they’re separated by a range of 15-20 pages, though in a tight script it’s not unheard of for them to be ten pages apart. In any event, there need to be three major developments in the story that build on each other, with the third development being the end-of-Act-Two climax. Usually, this is the point where things are at their worst for the hero. It’s sometimes called the “all is lost” moment because it happens when the odds have been stacked against the protagonist and everything that can go wrong, has.
The third act then usually begins with the hero somehow rallying as he prepares to snatch victory from the jaws of defeat. Then after about 15-20 pages, we reach the climax, where the central problem of the film is resolved. After that, there’s usually a coda, which can take anywhere from 5-10 pages and brings closure to the story.
I’m always leery of citing page counts because that gives the impression that scripts are completely cookie cutter and a writer is going to get called out if their inciting incident is on p. 16 instead of p. 15, or if the first turning point comes on p. 31. Don’t worry too much about that. Worry about the fact that if your reader is noticing the page numbers, then the pacing probably isn’t as tight as it needs to be. When a story isn’t flowing well for me, or the beginning of a script is dragging, 99 times out of a hundred, I can trace it back to the fact that the writer is taking too long to get to the turning point. Cutting down the excess almost always results in the turning points ending up roughly where they need to be in terms of page count. Pacing and structure go hand-in-hand, and there’s definitely a reason why reviewers notice when the first act runs 35 pages long.