I've been working on a drawing this week that has me thinking some more on the topic of back-story. I"d decided to draw my background and my characters on separate sheets, then overlap them. Of course once the overlap happened, a lot of details disappeared. Some of them were favorite details. I'd spent a long time getting them perfect, and in the final picture, no one will know they ever existed.
Backstory is a lot like that drawing. You spend a long time inventing a mythic past, or figuring out the floorplan of a castle, and then the story takes a differnt twist and the castle never appears, or the myth just bogs everything down and has to be cut.
When that happens its easy to feel like you wasted a lot of time, but you haven't, any more than it was a waste to lay out my picture on two different sheets of paper. The bits that don't show are still affecting what's around them. The shape of the room is different because of the stove that's almost hidden by my figure's dress. The shape of my invented society is different because of that myth that never actually made it into the story. Sure, it's a lot of work, but the effort shows -even when it feels invisible.