However, all of my scripts have something in common: I plan as much as possible before beginning to write any dialogue.
In interviews, Neil Simon has said that sometimes he plans Acts I and II, and then when he gets to Act III, he lets the sparks fly and sees where the script has gotten to at that point. Many movie directors have general ideas of where the script is going, but also are open to the serendipity that may occur during the shooting.
Others, like William Saroyan and David Mamet, have very specific ideas of where they want their works to go, what the actors will say, and how it will end.
I have found it best for me to work from some kind of an outline. When I write mysteries, I need to know from the beginning how it will end so the clues and various paths can make sense in the end.
Many scripts have a Chekov feel, that is, at the beginning, characters are in a certain state. During the course of the show, they attempt to free themselves from the state, but are unable to achieve success. In the end, they find themselves still in that state, but perhaps a bit wiser. Such a script would also need to know the end from the beginning.
But many other scripts can take off in numerous directions. Nearly every scriptwriter I know has had the situation where what was originally envisioned took a large left turn somewhere and ended up where it wasn't intended.
I think one of the biggest keys to overcoming this problem is with good planning. I recently mentored a teenage girl who was writing a short script for a full-year project. When I first began working with her, although she has considerable acting experience, she had only some random ideas and vague concepts of where the script would start, what it would cover, and how it would end. Sounds like many scriptwriters I've known.
But, as we worked over the months, we went through a process of planning, outlining, defining characters, scenes, and settings, collecting snippets of character descriptions and dialogue - all before writing any formal dialogue. Then we went through a couple of readings so she could hear the dialogue, and then based the rewrites on what she discovered. Glad to say, she received a 100% as a final grade.
While the specific steps may change from script to script and from writer to writer, those are basic components upon which to build the foundation for a solid script: creating a plan and following it to the conclusion.
