So, do I do that with my characters? Not exactly. Keep in mind that my brain works with complete concepts, it doesn't function like a normal brain at all (big shock, right?). My approach is a blend of the two. I come up with a character and add until I feel I have enough to work with, that I have a good idea how he or she will react to situations or what they will do, then tweek them and the main plot. I've seen some truly anal backstories by aspiring writers. There is such a thing as overkill. And the thing is, bios can be a detriment. You get so focused on the character and what he is supposed to do, that you wind up adapting the story to make the character work within it. And this creates a breeding ground for plot contrivances and continuity errors, because you are using the character's bio as the foundation to build on instead of the situation. For example, the movie "Orca," about a killer whale stalking a captain who killed his mate. The movie was so busy with its depiction of the captain dragged into the Ahab-esque situation, it ignored the fact that he could be safe by simply moving 200 miles inland. Problem solved.
When writing The Supremacy, I had rough ideas for the characters, how they behaved, and so on. As I started writing it, I was not onl adjusting the plot as I went because certain characters would not do certain things, but also adjusting the characters, to make sure they had a reason to participate in the proceedings. With all the deals and double-crosses going on, I had to make sure it was what the characters would actually do instead of something they do because its necessary to the plot.
A more extreme example is Raff in Stress Puppy. I was talking with some geeks, and one of them mentioned he worked for a "stress puppy". I had never heard of that before, and he explained it was a manager who thrives on crisis, and if there isn't a crisis, he will bug everyone until one pops up, usually through his neglect from misapplied focus. The image of a dog in a business suit having a mental meltdown popped into my head. It made me chuckle and I filed it away for "some other time". When I thought about creating a comic strip, I eventually remembered the "stress puppy" and thought it had potential, and started working on it.
At this point, the stress puppy (I hadn't given him a name yet) was the head of a department. And, unlike what you see in the strip now, he was a true stress puppy. Not only was I having trouble coming up with conflict for the character (as a department head, I was really straining to have his department intermix with the others), but he just wasn't satisfying to write for. He flips out. That was his shtick. The series nearly died right there because I just couldn't do anything worthwhile with him.
Then, I thought it would be easier to inject him into proceedings if he was a middle manager. Suddenly, he became a mediator between upper and lower management, between departments, whatever conflict the story I was writing called for. But that meant I had to change his character. A stress puppy is not going to be made a middle manager. Someone aware of the situation and thinking on his feet, sure. But not someone who flips out and makes everyone he works with so frustrated that they start flipping out, too. So the character's bio and personality underwent a complete rewrite, and the results were much more solid. I could not only write for the character, I wanted to write for him. Eventually, as other strips were written, more of his personality came out and became defined. For example, how someone who isn't a hustler can keep from getting screwed by people climbing the corporate ladder. Raff became an expert at gathering information from trusted sources and knowing just what situation to set in motion (plus how to do it) to emerge victorious. He became a realized character, but only because I was flexible instead of following the original map I laid out for him. Other details that I felt derailed the strips' pacing (Raff was originally supposed to have a secretary) were ditched in favor of things that enabled the situations in the strip to flow (Holly becoming his primary confidant and partner in crime).
So, my opinion is -- bios for characters are interesting, but you have to be flexible. You have to adapt. If something isn't working for you, and you're the creator, it's not going to work for the fans. Write your bios with pencil, don't carve them in stone.