Lighting is an essential part of telling a story. Light shows you where to look and enhances the emotional feel of each scene. In Finding Nemo, the lighting also helped create the feeling of being underwater.
Lighting designers light each scene to highlight the story and enhance the emotional impact. Pixar’s lighting designers have the additional task of defining virtual lights in the computer. They have to program in all of the individual effects of lighting such as where it casts shadows, how it reflects off shiny surfaces, and how is attenuates over distance in order to achieve the desired artistic effect.