The Art of Pixel Performers
scriptedfun writes "The BBC features the growing role of computer graphics in movies, but points out that it is still the human actors behind the CG characters which make them alive. From the article: 'It seems that the performance artist can still bring something to a performance, which [ a CG ] artist cannot.'"
That "something" is natural movement which doesn't trigger the Uncanny Valley reaction in viewers.
http://en.wikipedia.org/wiki/Uncanny_valley
The main difference between Pixar (from what I've read around) is that most companies get the famous actors first and build the characters around them (see Shark Tale), while Pixar seems to go the other way around, they set the character and personality first, and then they start looking for an actor whose voice would fit the character.