The four Acts of a screenplay (Act I, IIa, IIb, and III) correspond to the four parts of a story where part I is about the hero witha problem, part II is about the hero achieving success in solving the problem, part III is when everything falls apart, and part IV is the final conflict between the hero and the villain. You should focus on each part to develop a mini-story, but part III is especially helpful in identifying the subplots of your story.
A bad movie is one-dimensional because the hero has one conflict to solve and the whole movie revolves around that one conflict. As a result, the other characters exist solely to help the hero and obstacles appear random with no rhyme or reason. Think “Clash of the Titans” and you’ll realize how a one-dimensional movie feels flat because it is flat.
What every story needs is not just one main plot, but multiple subplots that intertwine and further complicate the hero’s life. In Act IIb, every hero’s life starts to fall apart. This is where everything goes wrong, so the more things you have to go wrong, the deeper the conflict.
In Act IIb of “Tootsie,” Dustin Hoffman’s main problem is that he’s in love with Julie, but she’s already shut him out of her life, thinking Dustin Hoffman (as Dorothy) is a lesbian). That by itself would make a flat movie, but here are the other subplots that complicate Dustin Hoffman’s life:
Dustin Hoffman vs. Les — Julie’s dad is in love with her (him) and proposes marriage. How will Dustin Hoffman get out of that?
Dustin Hoffman a fellow actor — A lecherous actor keeps making passes at Dustin Hoffman, even to the point of following her(him) home and trying to force himself on Dustin Hoffman.
Dustin Hoffman vs. Sandy — Dustin Hoffman is treating an emotionally unstable friend poorly by breaking dates with her and leading her on.
Dustin Hoffman vs. his contract — As a success, Dustin Hoffman is now trapped as Dorothy Michaels and must find a way to get out of the contract.
Those are five major problems that Dustin Hoffman has to solve. While your story may not have as many subplots, you need a few subplots to make your story deeper, but more importantly, to fill in your story’s gaps from the beginning of the story to the end.
Often times it’s easy to create the beginning and middle. What’s hard is the middle, so if you can identify all subplots that will culminate in a near disaster by Act IIb, you’ll just have to work backwards to set up these subplots that can help fill in the rest of your story.
Working backwards lets you identify your goal before you get started. If you start at the very end, you’ll know your beginning. If you start with everything falling apart in Act IIb, you’ll know what needs to appear in Act IIa, which can help you bridge the gap between your beginning and end.
The point is that you want to make your hero’s life as difficult as possible in addition to the main conflict, so identify all possible sources of problems and then set up these problems earlier in your story. By knowing your ending and all the problems, you’ll know how you need to begin the first half of your story.