The Cliff Edge has an interview with Pixar’s Pete Docter, director of Up. Here’s an excerpt on the voice casting:
Casting voices is vital but Docter says at Pixar they don’t worry about big name actors. “We just think about character. We strip little bits of dialogue from films with actors we’re interested in and play the audio while looking at images of our characters. Some voices just seem to put perfectly, and of course Carl and Ed Asner were a great fit,” he says.
Christopher Plummer voices the great outdoorsman that Carl meets in the jungle, and Docter says casting the legendary stage and film actor brought about changes in the character.
“Our thought was that he was like Ernest Hemingway, a big brawly guy that you’d find telling stories in a bar,” he says. “When we cast Plummer, he became a bit more educated and refined. You work with the actor and try to write so it becomes believable and whole. Plummer is an amazing guy to work with. I don’t think I’ve worked with an actor who is so prepared and instinctive.”