Movie Hole has got yet another unique interview with the duo, that contains some new information about Up’s 5 year process. The chat highlights why Docter chose the characters of Carl and Russell and what went into their development. They also talk about the differences of Up compared to the other Pixar films, design wise and technologically speaking.
It’s a very enjoyable piece, but be warned, spoilers are present.
Update: Animation World News also participated in an interview with Docter and Rivera. The link leads to the second page, the first contains plot related spoilers. There’s even more juicy Up info in this one, but read with caution.