A script should naturally break down into three parts or 'Acts'. Ignore anyone who tells you that a script has five Acts, or seven Acts, or anything else. They're just showing off.
Now, in some ways the fact that scripts have three Acts is a bit of an industry secret. We do not - repeat, NOT - write 'Act One' in our screenplays. We just don't, okay? I've read a fair few scripts in which writers have done just this - actually written in the Act beginnings - which you must not do. It's giving the game away.
The easy way to visualise the three-Act structure is as a bridge. Preferably, a suspension bridge. One with two towers or supports which, between them, hold up the middle section of the bridge.
That middle section is your story. It's the fun part. It's called 'Act Two', and it takes up roughly half of your script.
The first part of the script - 'Act One' - covers the approach to the bridge. The final part - 'Act Three' - involves getting safely off the bridge at the other end.
There is a lot of engineering in screenwriting. It's not all art. If you tried to build a bridge without really planning it, the chances are your bridge would fall down, which would be embarrassing for you and unfortunate for anyone who happened to be crossing it. So, to avoid all that unpleasantness, we apply STRUCTURE. Two pillars to hold up your middle section.
One quarter of your script is 'approaching the bridge'. The end of Act One comes when you reach the first structural pillar. Fully one half of your script is 'crossing the bridge'. The end of Act Two comes when you reach that second pillar. The final quarter is 'back to the safety of dry land', and it ends when we all know how the story ended.
The approach to the bridge - Act One - is also known as the 'Set-Up'. If we remember the Absolute Essentials (The Story is about a CHARACTER who wants to achieve an OBJECTIVE but an OBSTACLE stands in the way) then it is on the approach to the bridge, in the first quarter of the script, that this information is given.
So - Act One is about explaining who this story's about, what they want and what's stopping them from getting it.
Refusing to leave Act One is a common mistake, both for the writer and for the hero. The writer must remember that the story hasn't started yet. The story is the middle part of the bridge. Don't spend your whole script not-quite-getting to the bridge. Act One is for setting up the scenario. And then give your hero a big kick up the backside to get them past that first pillar and onto the bridge. End of Act One. We're a quarter of the way through the script and the story now starts.
Once you, the writer, have set up the scenario (the Story is about a Character, an Objective and an Obstacle), your task now is to amuse yourself with it. Now that you've got your hero into this pickle, how many ways can you exploit the situation? You have half of your script in which to ENJOY exploring the hero's new world, making him/her fight to get what they want.
I once saw a medieval woodcut of people crossing a crumbling viaduct while skeletons fired crossbows at them. It was meant to remind people about how suddenly death can strike. To me, it was a bit like crossing the bridge. People will keep taking pot-shots at your hero.
A lot of writers forget how important it is to torture the hero. Let's not forget - the story is about the hero overcoming obstacles to achieve their desire or objective. We are often told that drama is about conflict. It isn't, quite. It's about the changes the hero undergoes while overcoming the obstacles which stand between him and his objective.
If we think of the Objective as 'getting to the other side of the bridge', well, there's not much of a story unless we make that process rather difficult.
Always make life more difficult for your characters. That's what makes drama.
So - you've worked very, very hard to get your scenario established ('set up') in the first quarter of your screenplay and now you and your hero are off. You're making your way through Act Two. You are crossing the bridge, trying to avoid the missiles and doing your best not to fall.
Halfway across the bridge, and halfway through your script, you'll come to the Mid-Point. We'll return to the Mid-Point some other time. Interesting things happen there.
Once you and the hero have crossed the Mid-Point, the 'point of no return', you're homeward bound. You just have to make over the rest of the bridge and you'll have got to the end of Act Two.
Act Three - the final quarter of the script - is, in many ways, a mirror of the first Act. To begin with, you had to approach the bridge. You weren't really telling the story - you were setting it up, establishing who your hero is, what they want and what's standing in their way. To finish off your script, you just have to tell everybody how it ended.
In the first Act, we will have learned, very early on, that there is something not quite right with the world. There is a threat, an undesirable state of affairs. In the final Act we will learn, more or less at the end, that a new state of affairs exists, and that things are, hopefully, a lot better than they were.
How come? Well, because the hero crossed the bridge. He faced down his fears (of heights, bridges, skeletons with crossbows) and made it from one side of the story to the other, from a less-than-ideal state of existence to a (hopefully) better one. He, or she, overcame the challenges to achieve the object of desire.
The Bridge is the best structure for a script. It's pretty simple. Beginning, middle and end. And it stands as a reminder that the story proper - the adventurous bit - really only covers half of the total screenplay. That's the middle half. The actual bridge bit, between the two towers.
Act One - the first quarter of the script - is about approaching the bridge. The hero's job is to realise that something is wrong with his world, something needs to be done about it, but it will, unfortunately, involve a rather scary journey. The writer's job is to ensure that we know who this hero is, along with the basic set up of the story (Character, Objective, Obstacle).
Act Two - the middle half of the script - is the fun stuff. It's crossing the bridge, with the wind in your hair, and - DON'T LOOK DOWN! OR BEHIND YOU! And - are those SKELETONS firing crossbows at us? The hero's task is to get across the bridge, overcoming every obstacle and life-threatening situation en route. The writer's task is to enjoy every second by making life as difficult for the hero as possible. Enjoy the ride.
Act Three - the last quarter of the script - hey, we're off the bridge. But we've still got a way to go before we're home and dry. The hero's job is to make it home safely, finally seeing off any last crossbow-wielding skeletons. The experience of crossing the metaphorical bridge will have changed the hero, turning him or her into a better, stronger, more enlightened person. The writer's task is to get the hero right off the bridge and to tell the people out there how the story ended.
Approach the bridge - cross the bridge - get off the bridge. Or: set up your story, tell your story, wrap up your story.
Beginning, middle and end. One quarter, one half, one quarter. Act One, Act Two, Act Three. It's as simple as crossing a bridge.