An introduction to the traditional three-act structure for feature films, explaining the transitions from act to act, the nature of the content for each act and some examples of the structure in action.
A useful description
Before we cover the three acts in detail there's a useful description that's worth sharing, not because it's perfectly complete but because it puts in mind the core differences between the three acts. In the first act you chase your heroes up a tree, in the second act you throw rocks at them, in the third act they get themselves down.
Obviously this definition only gives us the most basic of introductions but it is a useful way to think of the three-act structure.
The first act
The first act introduces the main characters and the universe they inhabit. By the end of it we should know three things: Who's the hero? What does he want? What's stopping him?
So how do we do that? Well our hero has about ten to fifteen minutes to explore his initial situation, learn what prevents his success and, critically, introduce himself to the audience through his actions. Heroes run into burning buildings, are modest, are kind, protect the weak, save kittens stuck in trees. In short, if you have a hero, make him heroic and do it on screen.
What if our hero isn't heroic? Well the industry actually tends to use the term 'hero' to mean villains as well, sometimes, so let's use the word 'protagonist' to define our 'hero' instead.
So think about that for a moment. By the end of the first act, about fifteen pages in to our screenplay, we should know all of the crucial establishing plot, all of the driving and opposing forces, we should understand the background against which our story is played out.
The second act
In the second act we throw rocks at our protagonist. Not literally.
Even though we know the underlying objective of the character already and we know the obstacles he must overcome, we now fit 70 pages of twists and turns, problems that individually create the excitement of the plot. It's in this part of the story that we really discover how difficult the objective is, throw increasing obstacles in the protagonist's path, really test his resolve.
This is actually where the main part of the narative goes. If you were telling the story to a friend, this is where you'd recount the events in sequence along with what you did to reach the point where you could force a conclusion. Here are the setbacks, betrayals, battles, and in screenwriting teminology 'conflict'.
The second act is the part of the story you initially think of. It's the 'events'. If you don't have a good grip on the events then you don't have a narrative and if there's no narrative then your characters don't have motivation.
The third act
In the third act we resolve the problem to a satisfying conclusion. Actually that's a complete lie. We don't do that at all. Our protagonist resolves the situation, we stand back and watch them win the day. Our story can end with fireworks, or quietly and calmly, but this is the conclusion to everything that's gone before and if it doesn't lead to understanding or transformation then the third act lacks punch. The third act should be around fifteen pages long and should be self-contained.
One more description
Another way of describing it is as if you were pitching the story to someone else. Think of the entire story in terms of the trailer.
Imagine our story can be described as: Person wants thing but obstacle. Person does actions while other problems. Finally person does action and outcome.
Each of these sentences describes an act of the screenplay. Most screenplays, in fact most stories of any kind, can be described using this pattern. If your story doesn't fit ask yourself why not?
That's what we mean by the three-act structure.