Backstory is the collection of past internal and external events culminating in the current emotional and physical state of your characters at the start of your screenplay. It is essentially, the character dramatic background that eventually unfolds during the course of your movie to add understanding and depth for both audiences and characters alike. It can be told through dialogue, flashbacks, voiceover, narration or visual exposition.
Backstory can be superior or inferior.
This occurs when the audience knows more about a character than, or before, another character. An example of this would be if we know a man is cheating on his wife, but his wife is blissfully unaware.
This type of backstory is the opposite, when the audience is kept in the dark while the characters act accordingly. This adds mystique and piques audience curiosity. If the said wife took a shot at her husband then the audience becomes confused if we don’t understand why. Eventually they must be told. The secrets unleashed in both your superior and inferior backstories must collide to illuminate your theme and force a metamorphosis in your characters.
The key dramatic function of backstory in your movie script is to provide motivation for your characters. To define their conflicts, goals and modes of action. Their actions must be commensurate with their motivation and belief system. Otherwise your story has no purpose and subsequent audience interest. A character can’t kill someone for no apparent reason.
Use backstory as sparsely as possible in your screenplay. If unsure, leave it out. Like any plot point in your script, backstory must advance the plot or reveal a new dimension of character, not weigh it down.
Always enter your story as late as possible and drip feed the audience with the backstory throughout your film, rather than front loading them. This mimics human behaviour, because we act on the basis of our past experiences and knowledge. We rarely fully explain all our actions until after the fact. That’s what therapists are for. We generally partially rationalize and plan before we spring to action because we need to figure some things out along the way. Characters, like humans, cannot be pre-programmed to consistently behave in specific ways with given stimuli. Why? Because we are emotionally driven creatures and this often conflicts with rational behaviour.