Every story has a backstory. It’s the crucial information that you-the-reader need to know if you’re going to understand what’s happening now, in the story’s present day. Or it’s the buried secret that shapes the character of your protagonist, or the skeleton in the family cupboard, or the Dreadful Thing that happened at college in senior year that nobody ever speaks of and nobody ever forgets.
Sometimes a plot only needs a bit of light-weight backstory work, somewhat in the nature of a trellis to support the ornamental vines of the action, the better to reassure you-the-reader that what you’re seeing has something underneath it to keep it fixed in position and to hold it up. Other times, the backstory isn’t just there for support; it’s the heavy-duty engine that drives the entire narrative.
But no matter the relative importance of the backstory, there is one thing that the writer needs to remember: What went on in the past of the narrative cannot be more entertaining than what’s going on in the present. Because if it is, then the writer might as well give up on the present-day portion of the narrative entirely and concentrate on writing about all the past-era stuff that’s actually interesting.