Here’s how to write a weak screenplay. Start with a brief, vague idea and start writing. Chances are extremely good you’ll write about events that don’t matter and dialogue that doesn’t say anything important. This is basically how far too many people start (but rarely finish) a screenplay.
A far better approach is to identify your story as much as possible before you even start writing. While novelists often write to define and explore their stories, screenwriters can’t afford this luxury. Screenwriters must know where their story is going before they start writing. That way they know how to add foreshadowing.
At the most basic level, know your plot, which describes what happens in your story. This can be as simple as your story’s log line. In “Die Hard,” the log line might be a single sentence that describes how one man must battle an army of terrorists alone in a skyscraper.
Knowing this lets you identify the place and focus of your story. In “Die Hard,” this means your story should stay within the skyscraper and that the hero must constantly battle the terrorists and not get distracted by trying to feed his dog or win the lottery, which are actions that have nothing to do with battling terrorists.
A log line can keep you focused on the setting (place) and focus of your story. Next, describe the goals of your main characters. That way you keep the actions of these main characters constantly moving towards specific goals.
In “Die Hard,” John McClane’s goal is to get back with his wife, which means killing the terrorists one by one. Hans the villain’s goal is to steal corporate bonds and blow up the hostages so he can get away. The goal of John McClane’s wife is to keep her identity hidden from the villain and keep everyone alive.
If you fail to identify your main characters’ goals, you’ll risk writing scenes that wander and appear aimless. This creates a confusing, dull, and ultimately pointless screenplay.
A screenplay must tell a story that takes the reader (audience) on a journey. That journey can be anything you want, but once you tell the audience what that journey is (horror, comedy, romance, etc.), then you must deliver on that journey’s promise.
Writing a screenplay is less about actual writing and more about knowing what to write about. Until you know what to write about, all the writing and fancy screenwriting software programs won’t do you any good at all.