A scriptwriting secret: taking it one step at a time...

I'm currently working with a script client who is agonizing over his strong sense that he's missing something critical in his plot outline for a screenplay--something that will be a key to his story lifting off and really hitting the bull's-eye.  In fact, it's been stalling progress on his project and has pretty much left him walking in circles around his writing room with little forward movement at all.  He wants to have as complete a road map as possible before plunging into draft (and rightly so), but is stuck as to how to make that happen.