Some writers let their scripts evolve almost entirely in the actual writing of a preliminary draft. They make their discoveries pretty much exclusively as they turn out pages--lots of pages. My dear friend N. Richard Nash, author of The Rainmaker and many other plays, once told me that he often accumulated hundreds of pages in his first foray into draft in his search for the actual play he wanted to end up with. Horton Foote worked in a somewhat similar way, discovering the inner workings of the story as he went along, whether it was a play or screenplay.
At the other end of the spectrum--the one I recommend, especially if you are early in your writing career--is the commitment to extensive pre-draft exploration of the characters' backstories which in turn triggers the invention of a workable story/plot structure of your script-in-the-making. For most writers, young and old, this approach seems to be favored. And for good reason.
As I've said before in this blog, writing a viable script is like building a solid house. You start with the foundation and slowly build upon it. What the buyer eventually sees on the outside is the finished edifice, with its siding and trim, its color and overall design. Inside they experience the layout of the rooms and the interior "feel." What they don't see is the infrastructure, the framing, the plumbing, the wiring, the foundation below the frost line, etc. But there would be no house to live in if these hidden elements weren't securely in place.
An MFA student of mine recently put it nicely: "It's always a good idea to go into the first draft with a strong sense of story structure rather than try and fix a mound of problems later." I couldn't say it better. You want to begin the journey into draft with a pretty detailed road map of your chosen route within reach. Of course, surprises will happen once you're on your way, but these should be welcomed and explored. But you always have your map on the seat next to you when you get lost and need to find your way back to the main highway.
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I'm the Program Director of the low-residency MFA in Writing for Stage and Screen being offered by the New Hampshire Institute of Art. Our next residency runs January 3-11, 2016 and we are now considering applications for starting the program in January. I'm also a playwright and screenwriter, producing partner in my production company Either/Or Films (The Sensation of Sight and Only Daughter), and a professional script consultant.