I read an article in Time magazine that said that even with their state-of-the-art software (programs that make Poser look like Pong) they still had trouble with the teenage girl's long hair. "It kept falling off her head". When they started the process, one of the animators complained that "We just can't do 3D hair realistically!" But it looks like they pulled it off. They also used an advanced technique that allowed them to manipulate the muscles underneath the characters' skin. Normally it's just skin and bones, but adding a muscle element gave them much more lifelike movement.