Theme and premise can be useful tools, but are less often used, even though they are evident in stories. Plot is the most useful tool and has been explained in some depth. Contemporary wisdom has it that characterization is the essential starting point for a screenplay and that story follows from character. However, many writers need some idea of what their story is about before they can even begin developing characters. For example, as people confront questions in their life they often choose to write about them.
Characters become a device for exploration.
This idea is central to the Writers Workshop pilot program with high schools, giving students both the motivation to write, and an avenue of exploration. Writing is an interactive process and there is no one best starting point for all stories. Building characters is an excellent beginning exercise from which stories can develop and exploration can blossom. The workshopping approach allows writers to both write the story and see the result through staged reading. The Writer's Guild can supply you with a list of agents, particularly those willing to take new clients.