For the most part, screenplays evolve two ways: plot driven (often called High Concept), when a writer plugs original characters into a tailor-made plot, or character driven, in which the plot is born organically from the characters, usually an unforgettable main character – think The Wrestler (2008) or Crazy Heart (2009).
So obviously, for character driven screenplays – the character is the key, so you better create some damn interesting, incredibly flawed, and memorable characters. In the research and outline stages of a character driven story, you must still follow Three-Act structure and the specific sequences within, but you should never decide where you character will go next, how your character will react, or what a character would say in a given situation.
If you’ve done your homework, really wrapped yourself within the character iceberg, and you know your characters intimately, the rest is easy. The character tells you. All you have to do is listen.