Every single line is in the service of plot or story. A feature film is generally somewhere in the neighborhood of two hours long. Which brings me to… 4. The screenplay you write is a blueprint for the movie that will finally play on the big screen or television.
A book is the final work of art, whereas a screenplay is the first stage of a collaborative movie-making process than involves dozens of people.
A few writers have gotten as much as three million. In a screenplay, your dialogue carries as much weight of the story as your action does. However, for a screenwriter, they construct dialogue within and around a scene. The hero faces the villain, who is at their strongest, and wins!
The villains seem unstoppable. The hero needs to decide whether to embark on this adventure or take this new opportunity. So the screenplay, including a screenplay written by William Shakespeare, is only a suggestion to higher-ups.
And you don't need a high powered agent to get the script to Brad Pitt. It functions like a mini-story, has a protagonist with a goal, and an obstacle the protagonist must face.