on Fiction University:

Readers don’t mind backstory—as long as it’s something they want to hear about.
My first novel was fantasy, so naturally, I had a lot of backstory. Every character had huge histories and shady pasts, and I couldn’t wait to share every last detail with my readers.
And it turned out about how you’d imagine.
Boring pages, no action, flashbacks that nobody but me cared about. It was a mess.
A person’s past is part of life, and everybody has one—especially fictional characters. But that past isn’t always relevant, even if it is interesting. Stopping to explain a character’s history tends to bog a novel down.
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Too much backstory is also high of the list of why an agent rejects a manuscript, and many advise cutting all backstory from the first 50 pages.
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A bit extreme, sure, but more times than not, the backstory hurts rather than helps a story. But with a little forethought and revision, (okay, sometimes a lot of revision) you can make your backstory flow seamlessly with the rest of your prose.