What's an exaption? Google the term, and you'll likely soon find yourself in the middle of a debate over evolution. If you look it up on Wikipedia, you'll see that it's a trait that evolves for one purpose, but develops another use. Bird feathers are an exaption, since they would have first developed for warmth, but later underwent natural selection to the extent that they helped the bird to fly. Some philosophers think our ability to speak is an exaption: first we developed a brain for learning patterns. When paired with an ability to identify and produce a wide range of sounds, it became an exaption -- a trait that evolved in concert with the vocal ability. The better those two traits worked together, the better the animal's chances of survival. And so on.
Basically, it's a way of talking about the evolution of complex systems. And I am hereby exapting the term 'exaption' for screenwriters. Anyone who writes knows that 'writing is rewriting' is no mere cliche. Evolution is the name of the game. And evolution isn't all about aiming for a single goal and honing and honing until the piece fits the first and only conception of it. Evolution is messy and chaotic and brilliant. As a writer, you owe yourself to listen to that. As a human being, you owe yourself the chance to revel in the alchemical process rather than wallow in your misery around it. Plus, you'll actually get something done that way.
Who among us has not scratched their head at this problem: you write a beautiful love scene. It works great -- just like you want it. Then you write a big fight scene -- and you're Shakespeare. Next you knock out this great little visual joke that should bring the audience right into the characters. And then you look at them back to back, and there's just something terribly wrong. There's something poignant and charming about the characters in the fight. Or the love scene is hilarious, given the context, and the one-off joke scene now falls flat. You fiddle. You play. Or, if you're like many writers, you ignore it. You refuse to see how one scene casts a very particular light on the next.
Another example: your action hero is way too talky, and you're digging through, looking for ways to bring it into line. You're looking for shorthands, or vocal mannerisms, or visual cues to knock it down. You rewrite the dialogues with all the right rules in mind. And then you realize that your action hero has suddenly become a barrel of monkeys with a knack for calling 'em as he sees them at the first possible moment. Not what you had in mind.
But it's quite possible that it's BETTER than what you had in mind. You might really need that humor in the dialogue to balance the endless stream of death every time your action hero enters the room. Your script might be full of funny love scenes and poignant, charming fights. Every script has a feel -- and if it's a good script, it's a unique feel.
Finding the trait that's evolved without you realizing it and then tailoring it to your needs is a way of exploiting the evolution of the complex system that is your script.
Listening to your script -- how it REALLY reads -- is both difficult and necessary when hunting for exaptions. Not seeing the unintended humor won't get you anywhere. Not embracing the subtle interplay of two scenes won't help you. Looking at what tools your script offers you will always, always make writing a more enjoyable, organic process.
Don't think that you'll use everything that you find, or that every unexpected trait is a great exaption. There's no selection process as ruthless as rewriting. But next time you find yourself banging your head against a wall trying to fit round pegs into square holes, try stepping back and looking for exaptions instead.
And yeah, there's chaos here. You have to be a bit brave to accept the exaption. You have to have command of your script to clearly see what really matters. 'Seeing what matters' not infrequently means tossing your whole idea of a project. In Zoetrope, Francis Ford Coppola talks about his latest movie 'Youth Without Youth', a 'small' film, and how it arose after a torturous period of years trying to create a sci-fi epic.
Being willing to realize you've missed the important part takes guts. Realizing you've missed the part is a really shitty feeling. But writing what truly matters is worth the struggle.