For Pixar, making Finding Nemo was a thoroughly new adventure. For one thing, the majority of the action takes place underwater—someplace the animation team had never been. So John Lasseter made the animators take SCUBA lessons. At the same time, Pixar was moving to a new building and growing as a company. Here's a half-hour documentary about how Pixar made Nemo.
Sample quote: "Animating fish is like animating toilet paper underwater...but, like, muscular toilet paper! It's so loose and smooth everywhere, but muscularly driven, that it's a hard thing to do." -Dylan Brown, Supervising Animator, Finding Nemo
Starting around 14:45, The Ugly Contest, which explains why so many of the men interviewed in this thing have mustaches. This is how animation professionals party, people! Enjoy:
If you stick around past the 25-minute mark, you'll see some more animation/early visualization work. It's worth a look!