Most screenwriters have trouble writing dialogue that sounds natural. Either they write dialogue that vomits up information for the audience to know more about the story, or they write dialogue that serves no purpose.
That’s why you should study stage plays because stage plays must grab and hold attention solely through dialogue with minimal action. In a stage play, the purpose of dialogue is to advance each character’s agenda. In other words, every character who speaks is really trying to achieve a goal of their own.
In the following scene from “Argo,” the hero is trying to convince his CIA superiors to go along with his plan to create a fake Hollywood movie crew to go in and rescue people trapped in Iran. The hero’s dialogue exists solely to convince his superior to approve his plan while his superior’s dialogue exists solely to question whether this is the best idea available.
Because each character has a clear goal in that scene, their dialogue is laser focused. In comparison, most dialogue in scenes are written so poorly where characters have no goal, no direction, and ultimately no purpose for their words.
In “12 Angry Men,” the hero is trying to convince the other jurors that it’s possible that the defendant didn’t stab and kill someone with a specific knife. The other characters are trying to convince the hero he’s wrong. Because every character strives to achieve their goals (proving it’s possible the defendant is innocent or proving the hero is wrong), their dialogue is focused.
“!2 Angry Men” started out as a teleplay that was turned into a movie, but its roots as a play emphasize its dialogue where dialogue is used as a weapon by each character to get what they want.
Remember, dialogue never exists to give information to the audience. Dialogue always exists to help a character get what they want in a scene. Write dialogue that serves to get a character closer to a goal and you’ll wind up writing better dialogue.