I'm not sure how helpful this will be, but I play fairly serious "roleplay," but never start out with anything other than the barest bones of a backstory.
First - by "roleplay" I mean that my characters are complex individuals, with opinions and preferences and motivations of their own - not that I make them eat three times a day and sleep in a bed every night.
But those complex individuals grow into being rather than just springing full-form into it. I start out with just an outline (Imperial who talks his way out of things, for instance), then start browsing faces. A lot of times, I'll get some inspiration for more details from the face I end up with. Then I take them out of the sewer exit and usually just up the hill to the IC, and wander around there for a while. As they meet and talk to people, I'll start to figure out more details. Maybe something someone says or a particularly negative or positive reaction from a particular NPC or what-have-you will lead to another detail to add to the story. I just pay attention to those things and try to figure out how the character would respond to them. If someone's snooty and rude - does that offend him? Shame him because he feels insecure? Anger him because he thinks he's even better than the snooty person? Make him happy to find a kindred soul?
And it just keeps on growing that way. I'll just see things and think about how the character would react to them and they'll fall into place and the character will become that much more vivid, which makes it that much easier to figure out how things affect him. And along the way, in order to explain some of those ideas and opinions and preferences, I'll start working out whatever details from his backstory might be pertinent. And that way, I end up with a character and a backstory that are consistent, which just makes the character that much more "real."