To keep your story interesting, create a mystery and then gradually reveal that mystery. In Act I, you introduce that mystery. In Act IIa you deepen that mystery by providing more details. In Act IIb, you add yet more mystery. Finally in Act III you reveal that mystery.
In “The Lady in the Van,” the Act I mystery is a strange, homeless woman who lives out of her van and parks it in the driveway of a playwright. The mystery is who she is and why she does what she does.
In Act IIa, the mystery deepens as we learn that the homeless woman once studied music and is fluent in French. This new revelation shatters our initial perception that this lady is just crazy.
In Act IIb, the mystery deepens even further when we learn that the lady was once a nun.
In Act III, we finally learn about this homeless woman’s background as she used to be a nun and got kicked out, but still detained her love of music and her ability to play the piano.
The gradual mystery is often related to the villain, but the more mystery you can create with multiple characters, the better. In “Star Wars,” there are two mysteries. First, there’s the mystery of why Darth Vader is looking for when he captures Princess Leia (Act I). In Act IIa, we learn that Darth Vader wants to know the location of the rebel base so he can destroy it. In Act IIb, we learn that Darth Vader let Luke escape so he could track the Millennium Falcon to the rebel base. Finally in Act III we get to see that Darth Vader plans to wipe out the rebel base.
The second mystery in “Star Wars” revolves around Obi-wan. In Act I, we just think he’s a strange hermit living in the desert. In Act IIa, we learn he has special powers using the Force. In Act IIb, we see he has to confront Darth Vader. In Act III, we learn how he can use the Force to help Luke blow up the Death Star.
The mystery in Act I of “Zootopia” is why are animals suddenly disappearing around the city? In Act IIa, we learn that the missing animals are going crazy for some reason. In Act IIb, we learn that the missing animals are actually being held in a strange prison. In Act III, we finally learn why the animals are going crazy and why the villain is making them crazy.
What makes stories interesting is to get the audience actively engaged in trying to figure out the mystery of your story. That means creating a mystery and teasing the audience with that information a little bit at a time through each Act of the screenplay. Then in the final Act, you reveal that mystery so everything the earlier part of your story finally makes sense.
Create a mystery and that’s a huge step in the right direction for structuring your screenplay.