I'm working on that today for Jesse Spencer (I think he's a Spencer; names are turning out to be quite fluid in this story, with characters going back and forth across the white/Native line and gaining a new name just about every time they do, and for other reasons I'm going to keep off the record).
Jesse's history is taking me back to his birth and even a generation or two beyond that--all the way to the 1730s and the Scots-Irish migration to the Virginia and North Carolina back country. There are several crucial events back there that will have an affect on the plot in the present story (which is probably going to be set in 1787-88).
Writing a detailed narrative back story for a character is important to me, no matter how little of it ends up in the novel. I want my characters to feel like people who've had complex and interesting lives and been busy living them long before Chapter One opens a window into their world. I want to give the sense of dropping a reader down in medias res at a particularly crucial turning point in my character's life, one that's going to change directions for him or her, offer a new and enticing goal, or present some serious obstacle to overcome (preferably all of the above), but also one that flows naturally out of the choices they've made leading up to that point.
In other words, they aren't born into existence on page one of the story. Careful back story construction helps give my characters the sense of being fully formed human beings, with all the layers and depths a person gains through life--where they've been, the legacy they've inherited from parents and grandparents (material and emotional), what they've done, the choices they've made or had made for them.
So I'm off to write all those things about Jesse, and I'm sure he'll surprise me along the way. That's another thing I love about writing that's connected to this detailed back story weaving process. By taking the time to do this, I'm learning more about my characters than even I'm aware of, so that on the day I'm writing a scene and they say or do something I hadn't expected, still I'll sense when it's in character, or if it seems out of character why they've made the choice; I've taken the time to know them inside and out and if I look hard enough at their past, I'll be able to explain what motivated even the most surprising of actions.
in medias res, (Latin: “in the midst of things”) in narrative technique, the recommended practice of beginning an epic or other fictional form by plunging into a crucial situation that is part of a related chain of events.