This really impressed me today, from Steven Pressfield's blog:
The first Hangover ... is another great understory story. When the buddy-characters ... wake up in their Vegas suite with no memory of the night before but with a live tiger in the house, Stu missing a front tooth, and their friend Doug ... missing entirely, we know that the whole movie will be about uncovering the understory.
The concept of The Understory, as opposed to the Surface Story, won't be new to you, but the clarity with which he resolves it is excellent, and you won't forget how to mentally separate the two... probably ever.
(By the way if you haven't clicked the link because you're worried it's all writing tips about The Hangover, I can assure you it's not. Quite a lot of it's about Chinatown. So there.)
This Understory stuff is incredibly important because any time there's a process of discovery in a film you're writing, you'll be able to easily hold the two stories in separate parts of your mind simultaneously. You'll also be able to go through your mental database of stories and identify one with a close match for your Understory, which could be helpful in seeing how some master storytellers have dealt with your kind of story in the past.
(Working out the best use and integration of the Understory is really hard — so study the greats and let someone who came before you do all the heavy lifting.)
Incidentally if you don't recognise the name Steven Pressfield he's the author of two of my all-time favorite non-fiction books: The War of Art on overcoming resistance and its follow-up Turning Pro which applies the same ideas to helping you see yourself as a money-earning professional and lock that mindset in place once and for all. Links to both those are on the right of his blog.
He also wrote the novel The Legend of Bagger Vance which became a little-known movie directed by Robert Redford and starring Will Smith, Matt Damon and Charlize Theron. If you get a chance check it out, it really is quite something.
Happy writing, all. As usual comments very welcome.