I’m not really talking so much about the synopsis of a character you might do to place them in the events of story, give them at least the thinnest thumbnail of personality and motivation. I’m talking more about an in-depth sort of back story, more like chapters that won’t ever be used. You know they won’t but write them anyhow. I’m not sure there can be too much back-story, except that it seems like you might run the risk of losing focus. I certainly get side-tracked. A couple months ago, I got writers block pretty bad, and instead of trying to figure out why I got stuck, I spent about a week and a half developing verb conjugations for three dialects of one of my constructed languages. (See, there I went). Anyhow, I suppose many writers would write these in-depth histories as standard practice, I don’t know, it certainly didn’t occur to me as a new writer.
For each of two of my main characters, I’ve got what I would consider a chapter or two of events in their own history which serve to place them in the story for me. I wrote both of these as if I’d intended to put them into the story, with as much attention to setting and action as I might for any draft. In both cases they’re key to the plot of my story, but need to come out slowly to the reader as events unfold. So far, this approach feels good to me, because it helps me understand the character’s motives and place in a way that a few bullet-points or paragraphs in a notebook might not. It makes the events real for me as the writer because they happened. At the moment, I’m working on a similar chapter for a third character who didn’t get the same treatment initially, and I realized not having done that was a mistake even though the character really came alive as soon as she appeared. It left a big hole in the plot, which seemed like a nice thing to have at the time, you know for flexibility, but in filling that hole, the main plot gains credibility and I now have the basis for action that needs to happen in later chapters. When I finish it, I’ll continue revising from where I stopped, and in all likelihood I’ll be writing another of these when I introduce the next character.