Ten-year-old boys move ...

Ten-year-old boys move differently than middle-aged women, who move differently than athletic guys, who move differently than government bureaucrats.
 Brad Bird

Quotes from the same author

I think the industry tends to like to think in the narrow sort of mindset of a businessman, and businessman absolutes, and movies really exist in a much grayer region of dreams and stuff like that, and instinct is prized in movies, it's not prized with the businessmen in movies, but movies themselves often reward instinct rather than pie charts.
 Brad Bird
Even in hand drawn animation, humans are widely considered to be the most difficult to execute, because everybody has a feeling for how they move.
 Brad Bird
If you look at a lot of animated movies, they don't pay attention to how things move through space.
 Brad Bird
I think if you have a really big, heavy person, there's a feeling of an invisible puppeteer jerking them around in space. They don't feel like they are moving themselves.
 Brad Bird
If you move something 10 pounds through space and then stop suddenly, there's a little overshoot. When you transfer weight from one leg to another, there's a certain way that it happens.
 Brad Bird