Every scene in a screenplay must have a purpose. To make a scene interesting, it must advance the story and to do that, it must either setup an idea or pay off a previous setup.
For example, in “Die Hard,” the initial scene introduces the hero afraid of flying. That’s when a fellow passenger suggests he go barefoot and scrunch his toes in the carpet to reduce stress. That setup pays off later when he’s forced to run around barefoot.
That initial scene also sets up the fact that the hero is a cop and has a gun. This pays off later when he uses his gun to shoot some terrorists and take their gun. In addition by being a cop, the hero can believably fight with his hands and a gun later in the story.
Scenes must also emphasize an emotion because that makes the action in the scene more interesting. The opening scene in “Die Hard” shows the hero in fear.
In musicals, songs often emphasize the emotional state of the characters. In “Grease,” the song “Summer Nights” shows the happy emotional states of the two heroes.
Besides emotions, scenes must also show conflict. Even in the simple “Summer Nights” scene, there’s conflict when the leader of the Pink Ladies keeps dismissing Sandy’s tale of the boy she met as a bore. Meanwhile, the boys surrounding Danny keep badgering him for details of what happened.
Watch any movie and imagine taking a scene out. Then ask yourself, is it really necessary?
Watch “Mary Poppins Returns” and there’s a scene where Mary Poppins takes the children to see her cousin, Topsy Turvy, who breaks into song about flipping upside down. Yet this entire scene/song lacks conflict, emotion, and does not set up a future scene or payoff an earlier scene. As a result, the entire song is useless. If you read or listen to the “Turning Turtle” song lyrics, you’ll see that the song doesn’t affect the story one bit. Take this song/scene out and the story isn’t hurt. The scene only serves to introduce a character (Topsy Turvy) who plays no part in the rest of the story.
Think of any memorable scene in a great movie and you’ll see that it involves conflict, emotion, and setup/payoff. One of the most memorable scenes in “Pulp Fiction” occurs when John Travolta and Samuel L. Jackson are driving in a car, talking about fast food burgers.
There’s minor conflict as they all about burgers (which will pay off later when Samuel L. Jackson eats the burger of a young man who he’s about to kill), there’s the emotion of surprise, humor and incredulity about how Europeans eat fast food, and the scene sets up the fact that the two men are hit men.
Watch the original “Mary Poppins” to see the “Let’s Go Fly a Kite” song/scene that pays off the early scene where Michael wanted to fly a kite. Now in the “Let’s Go Fly a Kite” scene, the children get to spend time with their father, which is what they wanted all along.
Scenes are building blocks that rely on each other. They need conflict, they need emotion, and they need to link together with other scenes are either setups or payoffs. Omit one or more of these and the scene will be weaker. Omit all of them (think of the “Topsy Turvy” scene/song in “Mary Poppins Returns”) and you wind up with a scene that doesn’t help the story and simply detracts from it, weakening the overall story in the process.