There's an insightful interview with Brad Bird at AnimationTrip.com. Here's Brad talking about creating personality through movement:
One of my problems with full animation is, often times, people do beautiful movement, but it’s not specific movement. Old people move the same as young people, women move the same as men, and fat people move the same as thin people… and people don’t move the same. Everybody moves differently. If you guys were to just freeze the way you are right now, you’re both here, sharing this moment, but in very unique ways. [Brad looks at Chris] Like, you’ve got no shoes on but you’re nicely dressed at the same time. You’re kind of off to one side, but your head is counterbalancing it, tilted to the other side like that…it’s specific to you. I wish more people would pay more attention to how things move. I don’t mean just studying how things move…that’s important to know how weight shifts and all that. But it’s also the attitude of people. I feel that a goal that we set out in this was to have every single character have their own way of moving and have it be very specific. And even though it may change from animator to animator, we came to a very specific agreement on who this character was and what kind of moves they would do.