Flashbacks are scenes from the past that are inserted into a story to help readers understand what’s happening in the present, develop characters, or increase tension or conflict. Flashbacks are also helpful in memoirs and creative nonfiction as they can summarize important past events without having to tell the full history of what happened.
Length and Placement
Flashbacks can range in length from a few sentences to a few pages, depending upon the needs of the story. Since they contain backstory, you might want to treat the scene as a prologue, or begin the first chapter with it to avoid interrupting the action occurring in the present time.
Inserting flashbacks after the story begins takes events out of a logical, chronological order and can be confusing for readers. It also may break the flow of the story’s action or mood, so be careful not to place a flashback in the middle of an intensely emotional or active scene.
Tense Changes
Developing a clear transition from the present to the past and back can be difficult. If the story is mainly in past tense (he said, they went, etc.) use past perfect (he had said, they had gone) to show the transition into the past. Since the past perfect tense can become repetitious, you may want to convert to simple past tense after you’ve established the flashback is taking place. When the flashback ends, you’ll also need to clearly indicate that the scene is switching back to the current time period; another sentence in past perfect as the flashback ends may work as a good transition.
Ways to Show Flashbacks
Besides using a prologue as a flashback, or starting the story with one, you can insert bits of the past into the dialog between characters, as part of a dream the character has or talks about, through the character’s memories inserted as part of the narrative, or presented as letters the characters reads in the present time. Sprinkling a little of the past throughout the story may be less disruptive than inserting an entire flashback scene.
Done well, flashbacks can add depth to the story and convey important details. As with any backstory, though, you’ll want to avoid turning them into information dumps. Be sure they are vital to some aspect of the story, and keep them close to the scene in the present that they are meant to clarify. If the flashback can be omitted and the story still makes sense, you should probably leave it out.
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Do flashbacks taking place within a story annoy you? Do you prefer them as a prologue? How else can writers reveal something from the past?

