Your story takes place in the present, but at some point your character may flash back to a vivid detail or scene from the past. These glimpses of a character's backstory allow readers to connect with the character by showing the origin of that character's emotions or motivations. Begin with a narrative hook that presents a problem or sets the scene, introduce your character, then have him or her remember how the problem came about or what happened to put the character in that situation.
In his book Stein on Writing, Sol Stein says, "Flashbacks done correctly can provide richness and depth ... as long as they don't read like flashbacks, if they are active scenes slipped into and out of simply and quickly."
Your aim is to transition into the flashback scene smoothly. To accomplish this, use a triggering device, something that sparks the character's memory. It could be a sound, a smell, a taste, a touch, a familiar sight, an emotion, a line of dialogue.
This example is from my contemporary romance A Bed of Roses:
"Is that where you're from?" Dana asked.
Michael tried to shut out the memories her question stirred. "I moved up here from Los Angeles," he told her.
"What made you leave all that sunshine for Portland?"
It came back to him as clearly as if it happened yesterday--the pain, the anger, the humiliation. He was standing at the end of a long, polished table, surrounded by members of the hospital's administration.
"You understand, this is no reflection on your work," a stern-faced man with graying hair told him, "and I can assure you that you'll receive an outstanding letter of recommendation. But in light of recent events, we feel it would be best if you transferred to another hospital. Los Angeles General doesn't need your kind of publicity."
"Michael?" Dana tried again, asking, "Why did you leave L.A.?"
In this case, Dana's question triggers the flashback, which serves to push the story forward by exploring the why of the conflict.
Certain words should carry warning labels. "Had" mucks up more flashbacks than any other word. When writing flashbacks, use the same tense you're using for the present scenes, in most cases the straight past tense. Instead of saying, "I had been remembering," say "I remembered." Once you've established that you're making a flashback, jump in with both feet. There's no need to keep repeating had to remind the reader that you've switched time periods.
Pay close attention to the use of flashbacks in the books you read. Are the transitions in and out of the time shifts clear? Do you, the reader, always know where you are in the story? Experiment with flashbacks in your own writing, and decide for yourself, which method best tells your story and suits your style.
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Lessons From a Lifetime of Writing by David Morrell
Word Painting by Rebecca McClanahan
Thanks, But This Isn't For Us by Jessica Page Morrell
Begin with a smell that brings it all back.
The Pocket Muse: Endless Inspiration by Monica Wood