Telling The Wrap, Stewart says:
Right now my feeling is that the greatest innovations in cinema are being made in the world of animation. There’s such a diversity of work that’s being done. So when there’s a chance to take part in this new wave of great filmmaking, I like to take part in it.
He gets a free pass on the whole “celebrity voice-actor” thing because he is, in fact, a great actor in the classical sense who can add just as much to a performance with his voice as his movements.