One of the big differences between beginning writers and "old pros" is that beginning writers are always telling me how much they love sitting down at the keyboard and "winging" their scripts, while the pros invariably stress the importance of having a good outline before they start writing.
As an "elder statesman" myself, I've tried both methods, and outlining your story is the way to go. In television we spend as much time on the outline as we do on all drafts of the script combined. There are two reasons for this. Businesswise, if you outline every beat of your story and get the producer to sign off on it your relationship with said producer will be much more comfortable. And creatively, having a good outline means less work while writing the script. Instead of worrying about what happens next, you can concentrate on the most interesting and exciting way of making each event happen, and worry about clever, character-revealing dialog instead.
Take it from me. A writer's life like a chauffeur's. It's a hell of a lot easier if you know exactly where you're going and how to get there before you get behind the wheel.