The Story Behind The Story; or, Why Are You Telling Me This Now?

My WIP has taken a strange turn.

My brain has suddenly insisted that I stop what I'm writing and set down a (fairly detailed) account of the main character's eighteenth birthday. Chances are good that 99% of these scenes will never see the light of day. But I appear to have decided that I need to write it all down anyway.

I'm discovering, as I get deeper into editing some of my WIPs, that there are some scenes that need to be written not because they need to appear in the final story, but because I, as the author, need to assimilate that information about my character(s), and the best and most effective way to do that seems to be by writing out scenes that will never appear in the finished story.

The trick is to recognize the difference between the back story bits that need to appear in the finished work, and the ones that don't. In my current story, I know that there are two bits of information from the "birthday" section that do need to be worked into the final story. One is that the MC's grandmother, gave her and heirloom pendant for her eighteenth birthday, and the other is an interaction between her and her stepfather that occurred later that night and prompted her to move out of her parents' house the next day. I'll probably be able to work those bits in without too much trouble. But the other 3,500 words...well, I guess I'll save them for the "director's cut" version.

I've been having problems making forward progress on this story anyway (see previous blog entries), but the fact that this is happening now tells me that I have a problem with my story: I don't know my character well enough. Writing this bit of backstory helps me understand her and her motivations better, as well as those of her older sister, who is another key player in the story. I knew that the character owned a pendant that had been given to her by her grandmother. But now I know that:

- The pendant should, by rights, have gone to the MC's older sister. But it turns out that her older sister is really just her half-sister. Grandma knows this, but the MC does not. The trick will be for Grandma to justify her bequest without revealing the reason why the legacy is not passing to the older sister.
- The MC had more than sufficient reason for wanting to move out of her mother and stepfather's house when she did.
- The MC's sister was completely unaware of how the MC was being treated by their stepfather.
- When the MC left her parents' house, she moved in with her paternal grandmother for the remainder of the school year.
- The MC went to Iowa State University on a full scholarship, and majored in English. (Now if she would only tell me what she did for a living before she was laid off!)

Could I have finished writing the story without knowing all of these details about the MC. Sure, just like I could make spaghetti sauce without putting in eight or ten different herbs/seasonings and a tablespoon of sugar. I could make spaghetti sauce with just garlic and oregano, and it wouldn't taste bad. But with all of the subtle flavors added by the additional herbs and spices, it has a richer, deeper flavor--just like knowing all of this information about my MC will give my story a richer, deeper flavor.

How do other folks approach back story? Do you write it all into the story and edit it out later? Do you write it all down before you even start writing the main story? (I don't, but maybe I should!) Got any tricks for recognizing back story that doesn't need to appear in the final product?