Unity is a lofty word that basically means “don’t be arbitrary.” Life is arbitrary, scripts are not.
You know that old meme that goes “I see what you did there“? That’s writing in a nutshell. We always want to see what you did there and why you did it. Every element in a script should have a purpose and intelligence behind it, even if we can only see it after all is said and done.
Scripts are essentially patterns that are connected to a series of authorial choices. You have characters, themes, premises, settings and genres, all of which limit the possibilities available to an author. Consider Pulp Fiction – if Butch and Marcellus are in a room and they hear something outside, we know it’s not likely to be an orc, an alien, a time traveller, or Q from Star Trek. It’s going to be something that’s at least passably connected to the world of the story.
Most people avoid that obvious mistake and create a unity of genre, but dismally fail at creating a unity of sequences. If you can’t outline, break a story into 40 or so sequences, how can you break your scenes into a unity of beats?* Ideally, you want every line to be unified with the other one. Not every line needs to be astoundingly brilliant, not every line will be, but they should all be of a piece.
Oftentimes, I’ll give notes and someone will come back with a brilliant rebuttal that completely invalidates my note. Then I tell the person to make sure that rebuttal exists in the script, possibly as naked, expository dialogue, but hopefully exquisitely rendered in the gorgeous visual language of action. So the next time you’re reading something you’ve wrote, ask yourself – would the average person see what I did there? If the answer is no, make sure they can.
* The answer is that you do the best you can, write something, and work to build in unity later. Scripts are written inefficiently .