Daniel Manus, an L.A.-based script consultant wrote on the role of flashbacks in film scripts. Flashbacks (and flashforwards) are used to give an accurate portrayal of a time, event, or situation. Whether they are told through a characters’ own memory or told solely to the audience from a third person point of view, they express what actually happened. They effectively deliver backstory and plot.
A flashback is an interjected scene that takes the narrative back in time from the current point the story has reached. Flashbacks are often used to recount events that happened earlier or fill in crucial back story about a character or relationship. Using a flashback (or even a flashforward) can add suspense to a story or give us a greater understanding of what is going through a character’s head at a given time. It helps anchor the character’s emotional arc and clarify plot.
Flashbacks reveal or illuminate something we, the audience, need to know or something that pulls the story all together or progresses it forward. Flashbacks should reveal WHY an event happened or HOW it happened and not just THAT it happened.