Another helpful article about acting in animation, written by Doron A. Meir, can be found here. It astutely explains the primary elements for creating believable and interesting acting in animated characters. Having taken Robert McKee's screenwriting seminar and read his book, Story, (a book I HIGHLY recommend if you're interested in being a storyteller) I was fascinated by the similarities between McKee's and Meir's comments regarding inhabiting a character. It's this inhabiting of character that allows the creator to truthfully depict the choices and reactions that each individual character would have in that specific situation.
It seems that regardless if one is writing a character or drawing, the same concepts should be kept in mind. Creating is THINKING!