Chances are you've done your homework and you know what they ate for lunch, where they ate it, and with whom. You also know if their parents are alive, what their parents' occupations are or were, and whether they lived happily as a married or divorced couple. But just because you know that your character prefers Prada to Levi's, or vice versa, doesn't mean you have to share every scrap of backstory with the script reader.
Author James Scott Bell (The Art of War for Writers) talks about actively adding backstory to your writing (page 145). While Bell is writing to novelists, the same logic applies to screenwriters: tell us what we need to know to actively keep the story moving forward. In his example, from The House of Sand and Fog by novelist Andre Dubus, Bell notes how Dubus cleverly inserts backstory into dialogue. It's not a character running off her mouth, listing her characteristics: "I'm twenty-seven years old, a pisces, was born in Bermuda during a hurricane, had my tonsils removed," etc., but rather pertinent information that helps define who the character is and what his dreams are. It helps the reader (or film viewer) begin to understand the character's goals.
Another great bit of wisdom to keep in mind when uncertain about how much backstory to present is delivered by Director Sydney Lumet. In his book Making Movies, Lumet describes the early years of filmmaking when, about two-thirds of the way through a movie, someone describes the psychological truth that made the character who he was. He writes that he and Writer Paddy Chayefsky named this, "the 'rubber-ducky' school of drama: 'Someone once took his rubber ducky away from him, and that's why he's a deranged killer'" (page 37).
So while it's important that you know your characters inside and out, backwards and forwards, it's also important to know how to judiciously share relevant, character-building details that drive the story forward, not derail it completely.
Next blog post, we'll discuss how the character's wants and needs drive the plot forward.