Isn't it best (my creative writing course talking now), to write up character sketches, like, everything about a character....on a piece of paper (or however many it takes)....get them "out of your head" and onto paper....then just start writing the story....
Characters develop along with the story. It is probably best to write down the characteristcs as they appear. A character sketchbook, if you will.
Let's say you decide your character was born in the Winter. Jot it down.
Later when he is asked his birthdate, you can check your notes.
The problem for many is that trying to put all the traits together at the beginning is like shuffling cards or rolling dice for a role-playing game. It lacks a feeling of reality.