Upon rewatching The Prestige, my writer brain turned on after the second line. The one where Cutter (Michael Caine) explains how every "magic trick consists of three parts or acts." I dug deeper into this epiphany and what I found on the other side was quite cool, if I do say so myself. What I’m going to propose is another way of looking at the three-act format.
Your first chapter is a promise to the reader. For example:
The body lay in rigor mortis as I attempted to find more clues.
Simple, yes. But that sentence implies a murder mystery, albeit an obvious one. A “pledge” is another word for promise. The writer promises in the first chapter what the story will thematically be about. Theme in regards to whether the opening gives a sense of an action/adventure story, fantasy, horror, or science fiction. The Writing Excuses podcast did an excellent episode on the subject of fulfilling promises to the reader, which is what a good beginning should do: set-up the promise for fulfillment later.
The beginning of the second act is when the reader/viewer/listener knows that the problem is much more insurmountable than they were originally lead to believe. The first act is often a Macguffin in Hollywood movies, Thought it doesn’t have to be. The second act in a Hollywood movie tends to be the longest of the three, and the one filled with the most tension. Sometimes this act is called the “try/fail cycle” act, because things just get worse and worse for the heroes, until they finally succeed and the story can move onto the third act.
There are many ways to write a third act, but the one I try to strive for is a “full circle” ending. Depending on the story this can be easy or hard. It's mostly hard if you don’t know what your final scene is going to be. For example in my novel (Editing Phase) I knew how my protagonist was going to defeat the bad guy. However that kind of ending is premature. There needed to be another chapter. This is part of the reason why epilogues are still around. After a few weeks of staring at a blank screen, I thought back to the beginning of the book and asked what promises I was making to the reader. After a quick Q&A brainstorming session, I realized what I needed to have happen.
Act I and the Act III are mirrors of each while Act II can be used to misdirect the reader by making them forget about the events of the first act. This type of plotting technique is done quite well in Ocean’s Eleven (2001), where upon re-watching the film you notice that all the ground work was established. The danger of this method is that your clues can sometimes be too subtle. The first Mistborn book does this, I feel.
Another way to look at this is via the screenwriting term: “surprising, yet inevitable.” The writer made a promise you didn’t know was going to be fulfilled a the end, and you love that they did.
Check out my first novel with the second one right around the corner.