So, I've been working on some short stories about my two main characters - Anne and Charlotte - and I'm already reaping the benefits of these "background" stories. These stories help me rethink my characters actions, both why they act and react in the ways they do in the novel. Motivation is clearly a huge part of this, and, as usual, a useful writing exercise can come out of this sort of thinking: asking "why" questions about your characters.
Here are some questions I've been thinking about lately, as I revise my WIP novel...
Why does Anne have a difficult time talking to her best friend about her mother?
Why is Anne intent on "finding" her mother? This one seems obvious, in ways, but then...
Why does Anne have difficulty talking to her grand-mother about her mother's past, if she's so desperate to "know" her mother?
Why is Anne experiencing Charlotte's life? Why does Anne think she's experiencing Charlotte's life?
Why can't Anne grasp what's going on with the riots or with Pierre?
Why can't Charlotte live with her father?
Why does Charlotte refuse to share her true opinions about the revolution with her family (except for her father)?
Why does Charlotte believe that France should be a republic, rather than a monarchy?
Why does Charlotte believe she has the destiny she does?
Why does Charlotte decide to go to Paris and commit...
I'll stop there; I don't want to give too much away =). Although, hopefully my questions - all of which I answer in the novel, both in subtle and non-subtle ways - intrigue you. For me, at the very least, these questions have been useful as I revise my novel, making sure that how my characters act lines up with their past actions.
Try this yourself; ask 'why' questions of your characters and delve more into their motivations, goals, etc.