Character back story can be a pain.
Some people chart out extensive histories and genealogies for every bit part. (Tolkien, I’m looking at you.) Others simply conjure characters out of empty air, making them up as they go along. That’s probably unwise, but I tend to operate that way.
The problem is that characters are supposed to be people. People don’t blip out of existence when we turn our backs. They have their own lives, interests, problems… their own stories. Some awareness of this gives characters a touch more realism.
I’m very bad at this. To try to fix it, I forced myself to imagine a scrapbook of sorts of characters’ lives. What moments do they cherish, blush at, move beyond? What events shaped them? Sometimes, some detail stands out and I work it in. Other times, I tweak their background to explain their behavior in-story.
Back story building can be a thankless task because most of it won’t factor in to the story. What’s the point? I’ve been frustrated many times after constructing an elaborate personal history of a character that gives great insight into their mind and actions, only to realize there’s no place to add it in. I explained why the villain did a lot of what he did… and I couldn’t fit it in! Having someone launch into a speech about their childhood traumas in the middle of a fight scene doesn’t really work. At the other end of the spectrum, minor clues and comments scattered throughout the text might be misinterpreted or glossed over entirely. I can’t write a prologue entitled The Life and Times of Every Important Character Including Important Defining Moments That Will Come Into Play Later.
This is easiest to get around in third person omniscient point of view. Then the narration can include offhand comments about so and so’s past. Otherwise, you’re pretty much stuck with soul-baring rants. If you can’t fit one of those in… you’re in trouble. Maybe you should rethink tying so many important plot points to the main character’s sixth birthday party….