For a long time I held aloft, above all others, Michael Hauge's one-word answer to the question: What makes a screenplay work?
On the first page of Writing Screenplays That Sell, Michael explains:
People do not go to the movies so they can see the characters on the screen laugh, cry, get frightened, or get turned on. They go to have those experiences themselves.The reason that movies hold such a fascination for us, the reason the art form has been engrossing and involving audiences for close to a century, is because it provides an opportunity to experience emotion.That was then; this is now.
... All filmmakers, therefore, have a single goal: to elicit emotion in an audience.
Now, I understand there's something loftier. Something more seminal. It was there all along. I just never bothered to follow the breadcrumbs all the way to the source.
It's true: eliciting emotion in your audience makes a screenplay work.
So, how do you create emotion on the page? How do you make the reader, the filmgoer feel sad, mad, bad, or glad?
A boy goes to war. A woman sues her boss for denying her promotion. A dog journeys cross country to find its owner.
Raise a conflict and the audience will pick a side according to the bias of their own life experiences. They'll feel outrage watching the war turn the boy into a soulless, deadly efficient soldier, despite his oath to his father about becoming an army doctor and saving lives, never taking them. They'll laugh when the woman wins her case, gets promoted above her former boss, and gives him hell having turned the tables. They'll cry when, despite every devastating setback along the way, the dog staggers those final few feet into its astonished owner's loving arms.
Notice what happened in that last paragraph. I gave you conflict, then I gave you something far, far more elemental and powerful.
And here we are. Back where we started: What makes a screenplay work?
The boy and his father and the promise.
The woman and the boss who sues her and the reversal of fortune.
The dog and its owner and the incredible ordeal.
Conflict is meaningful only when there's a relationship.
Relationships channel conflict. Conflict elicits emotion. Emotions make your screenplay work.
More than anything during your story development, study and perfect your character relationships — to themselves, to the people around them, to the world.
Then SMASH your characters against each other with all the Creator fury you can summon.
* I use 'screenplay' throughout, but of course you can swap in anything else: story, novel, comic, pop song. Wherever you can plant a relationship you can generate conflict and make an audience feel something.