The secret of successful screenplays...

...is not structure, after all. According to Billy Mernit, "The most important task a screenplay must accomplish is to get whoever is reading it to identify with the lead character."

In fact, Mernit goes so far as to say that successful screenwriters tend to disregard the 'rules of screenwriting' (keep it brief, show don't tell etc) in order to make sure that readers identify with their lead character.

Sounds simple. And of course screenplays can't easily be split into 'character' and 'story', it's all intertwined (or should be). Nonetheless, it's an insight that strikes me as quite profound.