Holly Lisle has a great article on How To Create Characters. She may be talking about writing for novels, but the same principles apply.
She doesn't think about their physical appearance. Instead she starts out by giving her character a problem, a dramatic need, a compulsion. That's your springboard. An insight into their internal landscape.
Bill Martell writes about 3-D Characters: using contrast to explore character.
There are about a gazillion other articles online about creating character. If you have any you want to recommend, please give details in the comments?