Actors come in, and they have their own take on things, and you have to adjust on the fly to make sure everything still works structurally and dramatically.
I'm a huge fan of 'Kung Fu Panda.' I honestly think that Jack Black's voice work in 'Kung Fu Panda' is the best voice work that's ever been done. He's so funny, and it's such an endearing character. It really is great.