For many years Syd Field worked as a script analyst for several major studios in Hollywood. After nearly twenty years of reading and recommending numerous scripts that turned into numerous award winning movies, he realized that the best of the best seem to follow the same basic format. And that format was what he called the “paradigm” that included three acts (setup, conflict, resolution).
Anyone who has watched a standard two-hour movie on the big screen realized very quickly that the first thirty minutes usually sets up the story and introduces the main characters. The next hour or so develops the story in depth and follows the main character’s struggle to either solve a mystery or achieve a specific goal. Then the last thirty minutes contains the final struggle and climax to the story. Thus, three acts.
Now, do all movies have to have three acts??? Of course not. But then not every film maker wants or cares whether she has an audience that follows and understands her story. So, a person may write whatever way he wants. But, the filmmaker that wants people to watch his movie, tell other people to watch his movie, and have people think his movie is really good… well… I think you get my drift.
There is nothing wrong with tweaking the structure and making it do what you want it to do such as is the case with films like PULP FICTION and possibly RENDITION. But even those movies easily flow from set up into conflict and end with a resolution of some kind wherein the story being told is done.
In reality, screenwriting is more a skill than an art form. But like any skill, it can be used as an art form by an artistic genius. Unfortunately, there just simply are not very many artistic geniuses in the world.
So, "Good writing to you! And go out and watch a movie this week."