Flashbacks can sometimes pull audiences out of the story, depending on their length and frequency. Other times, flashbacks can be effective. However, I've noticed that sometimes writers use flashbacks so often, and believe me, I've done it myself, that it draws audiences out of the story and quickly loses interest. Is there a number of flashbacks your story can have? No. Personally, I believe that flashbacks should be there to either provide insight on a characters backstory or an event that pushes the plot forward.
How many times have you seen an action scene, cut to a freeze frame followed by a record scratch, and the protagonist says, "You might've wondered how I got in this situation"? In that particular example, it rarely happens, but I recently read a script where someone has a freeze frame followed by a lengthy flashback, which, for the life of me, I couldn't tell when we were back in the present moment.
The key thing to keep in mind is formatting. There's two ways to format flashbacks.
Formatting #1
BEGIN FLASHBACK
(Scene heading)
(scene)
END FLASHBACK
Formatting #2
INT. LABORATORY - NIGHT (FLASHBACK)
(Scene)
EXT. CEMETERY - NIGHT (BACK TO PRESENT)
There's also a "MEMORY FLASH" which is a brief flashback before we return to the present. Ratatouille does a great job using a memory flashback because, after taking a bite of Ratatouille, we briefly flash back to his childhood, where we see him eating it for the first time, then return to the present with the same expression.
Making a flashback effective
If used properly, flashbacks could be effective to your film. Take Forrest Gump, for example. Most of the film is a flashback because we watch Forrest's life from childhood to adulthood, navigating various historical events. It's great because it moves the plot forward.
So, before placing a flashback into your script, ask yourself the following:
Does this advance the plot?
Do we get a better sense of the background of the character or event?
Am I oversharing?
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