"Ratatouille," Pixar's new film about one rat's dream of haute cuisine, came by its acting chops the old-fashioned way, turning to real-life chef Thomas Keller for the inside dish on kitchen kinetics.

Ask me my favorite meal and the answer will vary depending on the time of year. Right now, I’d say ratatouille. I can’t think of a dish that better exemplifies the taste of summer. When I eat this ...