As a plot evolves, the writer needs to decide how to spoon in backstory (exposition), which involves breaking from the main storyline to give the reader information relative to the plot. But too much exposition can be deadly. Backstory, either in dialogue or narrative, should be kept to a minimum and not continue for more than a page unless the plot contains an historical subplot.
Well-handled exposition provides perspective, dimension, and needed context to connect previous events with the present. A micro history, if you will. But exposition should be handled carefully. Comic strips and soap operas use the technique frequently, but novel characters should rarely use exposition to reveal past events, especially if they’re not relevant to the plot. Mystery novels should be written in a straight line with the story’s conclusion in mind, as though running a marathon with blinders. Forget those spectators on the sidelines. Always look toward the finish line.
An example of unnecessary exposition:
“Remember that key you gave me?”
“Which key is that?”
“Tied with a yellow ribbon.”
“No, I don't recall . . .”
“You said it was the key to your heart.”
“Was I drinking champagne?”
“You dropped the key in my slipper.’
“Come now, Carla, what’s the point?”
“Nothing, David. Nothing at all.”
If the above dialogue is a lead-in to a romantic scene that's central to the plot, it’s okay, or if Carla is planning to kill her lover. Otherwise, it should be deleted. An agent once called a similar conversation nonessential and advised me to revise it because it added little to the plot. Dialogue that doesn’t contribute significantly to the plot should be eliminated, no matter how much you like it.
When the first draft is finished and polishing begins, eliminate conversations that don’t characterize, provide limited exposition, or carry the plot forward. Any asides, cute expressions, and nonessential chit-chat need to go, no matter how cleverly written. Save them for the next novel and build a plot around them. Editors consider nonessential dialogue “padding,” and if the work is accepted in spite of padded prose, the copy editor will delete it for you. It’s better to do that yourself.