I create them by starting with the most obvious things that are visible, and work towards the less visible or less important traits and details, basing it on the more important ones. It's a bit of a reverse psychoanalysis.
For example, the character I want will need to be someone independent and charming, but a bit too nosy and who tries to stand out too much.
I'll take that part and say, they should probably be shorter than average as they like to prove themselves. Charming and nosy makes them perfect to either work as an assistant to someone, customer support or a social worker. Or, if I want them to hate their job, make it something that's the opposite and where they can't be nosy at all. And depending on their part in the story, I'll pick a more specific profession.
Independent makes me want to make them have either irresponsible or no parents, or regular parents but the person is an introvert. And if a person is cheerful, I make their birthday in summer or spring, if they're more broody, or they're overly optimistic even in the worst of times, I make it winter. And so on.
Again, with a more specific story in mind and their part in it, the choice becomes easier.
There are a lot more choices, of course... and the person above could've easily been any height, or have any job, so nothing is set in stone. Just need to make sure that whatever you pick, that it fits in in your mind. And as long as that works, it should be easy to work with the character afterwards.
And I just continue this till I'm satisfied with the background and the person's life, even if 90% or more won't be known to the reader or appear in the story. Overall, it doesn't take me long to create a believable background as long as I know the part I want the character to play, and their general behaviour.
Last edited by ekster; 11-06-2012 at 05:59 PM.