Normally, when I develop a character, I figure out the following things:
arrow What does this character want, and what is his/her motive?
(The reasoning behind this is purely for plot.)
arrow What are they willing to do to get what they want?
(Will the character stomp all over everyone else or find a more practical means to achieve their goal?)
arrow What's their alignment, and why?
(Believe it or not, but even in real world situations, people have an alignment. Some are bad, some are good, some are horrible, and some are amazing. There's a reason behind this. Some were not taught consequences of their actions, while others were abused, mistreated, or suicidal. Of course, these are extremes,and as most of us know, some choose to be the way they are, despite their upbringing.)
arrow What's the basis of their nature?
(Are they introverted or naturally outgoing? Things like this can help provide the basis of dialogue and actions.)
arrow What are their strengths and weaknesses?
(The reason for this is to figure out exactly what they're good at and what they're bad at, which helps flesh out personality.)
arrow What are their best characteristics?
(These should be legitimate reasons of why someone would like them.)
arrow What are their flaws?
(Flaws make a character more realistic.)
arrow What's their body type, height, weight, and possible handicaps?
(This determines physical limitations. Handicaps are not necessarily medical in this case. Does a character have trouble jumping? That fence isn't going to clear itself, you know. That character better find a way around. Things like that are helpful for when characters are placed in certain situations, everything from gym class to murder.)
arrow Do they have a specific quirk?
(These are important, as they help a character seem more realistic.)
arrow How do other characters perceive this particular character?
(Normally, I take this into a bit of a character development game. I write at least a few paragraphs on what each character who knows this particular character thinks of her/him and why. Not every character will have the same opinion, and even they do, it's generally for different reasons.)
arrow Fight or flight?
(What's the reaction to situations? Does the character run from problems or take them head-on?)