A good story should be like a puzzle. Initially the story may be confusing but intriguing. Gradually as the story unfolds, you start to unravel tidbits of information a little at a time until you finally understand everything right before Act III. Then Act III is all about action where the hero confronts the villain.
There are two ways to provide gradual realization in a story. First, you can gradually reveal the villain’s goal. In “Star Wars,” we don’t understand initially what Darth Vader wants. Gradually as the story unfolds, we learn that Princess Leia hid something in R2D2, that R2D2 is trying to find Obi-wan, and that R2D2 has the plans tot he Death Star that the rebels need to defeat it. By gradually revealing information about the villain’s goal, the story keeps our attention by feeding us additional clues until we finally understand that Darth Vader’s goal is to find the rebel base and destroy it with the Death Star. We also gradually learn what Princess Leia was trying to do all along.
Besides gradually revealing the villain’s goal, a second approach is to gradually reveal the hero’s background. In “Snowpiercer,” the hero is leading a ragtag group of people to take over a train that contains the last remnants of humanity. (SPOILER ALERT. If you haven’t seen this movie and plan to watch it, don’t read any more.) The villain’s goal is clear from the start: stop the poor people in the back of the train from getting to the front. However, the hero’s background is a mystery, which gradually gets revealed.
First, we learn a hint that the hero doesn’t consider himself to be a leader. Then we learn that he has a scar on his arm. Next, we learn that his friend doesn’t remember his mother. Finally we learn that the hero doesn’t consider himself to be a leader because he killed his friend’s mother and was planning to eat her baby, which eventually grew up to be his friend.
The hero also reveals that an old man cut off his own arm to feed the starving people so they wouldn’t eat the baby. Then other people offered to cut off their arms to feed people, but the hero couldn’t do it, which explains the scar on his arm. Because of his guilt in killing his friend’s mother, the hero has never considered himself to be a leader since he’s ashamed of his past.
This gradual revelation of the hero’s background makes the hero feel more real as we understand his motives. In “Thelma and Louise,” we also gradually learn about Louise’s background that she was raped in Texas and that’s why she hates rapists and Texas. When we fully understand the hero’s background, it makes the hero more sympathetic.
So in your screenplay, look for ways to gradually reveal the villain’s goal or the hero’s background. Either way will work to keep your audiences attention as you gradually reveal bits and pieces of information that keeps them wanting to know how it all turns out in the end.