The whole thing about writing a play is that it's all about controlling the flow of information traveling from the stage to the audience. It's a stream of information, but you've got your hand on the tap, and you control in which order the audience receives it and with what emphasis, and how you hold it all together.
I feel that when I began writing, I had a need to know more about the play before I got into it. I think that's the way I was thinking. But my actual experience is that the best way to find out what the structure is, is by writing the play out laterally. You just have got to be brave enough to start without knowing where you are going.
One of the nice things about the world of filmmaking is that you make friends in the business. Sometimes directors feel a script needs something, but they're not sure what it is, so they show it to a friend; if the friend is a writer, he ends up kicking around with that script for a while.