Gromit is one of my favourite characters ever created in animation. And he never speaks. Yet there's so much expression in him.
It's all about body language and the way his face presents itself to the camera. And making your audience understand what he's thinking without saying a word - it's so difficult.
I've seen less capable comedic human actors than Gromit. What he can do with a look is sort of Stan Laurel-esque. It's approaching genius, if not actual genius.