Carlos Baena (Pixar Animator):
So...between a good animator and a great animator. For me, it's the difference between a good actor and a great actor. The animation skills themselves, with time, you learn them little by little. The acting part of things...the choices you have your character do. That's where the true skill relies. I think it takes a lot of studying to get there. Not necessarily animation studying at this point...but acting/drawing/craftmanship/mime/comedy studying. Also your personality has to do with it sometimes. Something you think may be funny, may not be necessarily funny for other people. It's tough. But in the end, and to answer to your question, it's all on the acting. Some animators at Pixar, I don't think of them as animators anymore...I think of them as actors. Purely. Mark Walsh is a great example for me. He has done some of my favourite acting shots in this company, and the best part, his acting is sincere and comes from his heart and experiences. That's what blows me away personally.
Source : Shaun freeman animation tips