Just as a train carries passengers through a landscape, a screenplay carries readers through a narrative. The story line, made up of the premise and the obstacles a protagonist will face, keeps the screenplay on track. The "stops," or points of arrival, along that story line are story events and major story reversals and, like real stops, are subject to schedules and estimated times of arrival. The movie's engine, that element which gives the story its forward motion, its sense of pull from scene to scene, is dramatic tension. A real train cannot jump its tracks or skip its stops, and neither can a screenplay jump its story line or skip its major reversals.
To be effective, dramatic tension needs to be offered up at the right times and in the right amounts. Too little tension results in a lack of motion; the screenplay may leave the station, but it won't get very far. Too much tension can be overwhelming; the risk is a runaway screenplay. Badly organized tension is confusing; the audience will wonder what the destination is.
How does a screenwriter go about organizing dramatic tension? A good place to start is with the three-act structure.
Authors of full-length plays have been employing the three-act structure since its origins in ancient Greek drama. The three-act structure is also the convention in Hollywood. While it is not the only successful way of organizing a dramatic narrative, all serious screenwriters should become intimately familiar with the principles of the three-act structure. If nothing else, understanding what studios expect will benefit any fledging writer.
What is an Act?
Before we can understand the three-act structure, we need to define the term act.
An act is a series of scenes and scene sequences that work together to build toward a major reversal that is, a turning point powerful enough to spin the story in a new direction
Let's look at an example. If we write a scene where a detective interrogates someone, the scene's climax will occur when the detective either has or thinks he has the information, or fails to get it. Things shouldn't be too easy for our protagonist, so we might write a few scenes that end in failure or work to mislead the detective before we finally allow him to uncover a genuine clue. Several scenes that work together like this to build tension toward a bigger climax (in this case a genuine clue) are called a scene sequence. If we continue in this way, we may eventually get to a point where our detective realizes his client, who has until now played the victim, is in fact the perpetrator. This information is so new and shocking that it completely changes the protagonist's point of view and spins the story in a new direction. This is the major reversal. Each scene and scene sequence climax brought us to this major reversal logically and organically, so the new information, when revealed, is both surprising and believable.
What is the Three-Act Structure?
The three-act structure is an organizational tool that uses three acts as a framework to plot out dramatic tension and tell a story. A three-act structure has three major reversals-one at the end of each act. (The major reversal at the end of Act III is also referred to as the climax. Major reversals are also called turning points, plot points, or act climaxes.) Major reversals are the surprises or twists that keep an audience glued to their seats. We expect good stories to have major reversals and are disappointed when they don't. Movies are substantial works that run 90-120 minutes, or sometimes more. It will take a minimum of three major reversals to keep the audience involved for that length of time. In his book Story, screenwriting guru Robert McKee states:
"In our effort to satisfy the audience's need, to tell stories that touch the innermost and outermost sources of life, two major reversals are never enough. No matter the setting or scope of the telling, no matter how international and epic or intimate and interior, three major reversals are the necessary minimum for a full-length work of narrative art to reach the end of the line."