Casting

As inspiration for the crowd scene in the film, the production staff spent a long time looking at the Simpsons poster that features over 320 of the show's characters.Groening said that they did try to put every single character into the film, with 98 having speaking parts.The series regulars Dan Castellaneta, Julie Kavner, Nancy Cartwright, Yeardley Smith, Hank Azaria, and Harry Shearer, and semi-regular performers Marcia Wallace, Pamela Hayden, and Tress MacNeille reprised their roles.Joe Mantegna returned as Fat Tony,while Albert Brooks, who supplied many guest voices in episodes, was hired as Russ Cargill, the film's antagonist.Brooks told the staff that if they were doing a film, he would like to be part of it, and was brought in. The cast began recording every week from June 2006 onwards,with James L. Brooks directing them for the first time since the television show's early seasons. Castellaneta described the recording sessions as "much more intense" than recording the television series, with them participating in "20 or more" sessions, and more emotionally dramatic than the TV series. Some scenes, such as Marge's video message to Homer, were recorded over one hundred times, leaving the voice cast exhausted.

Green Day played themselves in the film.When the scene was being written the band was not specified, but the staff received a letter saying that Green Day wanted to guest star in The Simpsons, and they were given the part. Tom Hanks also appears as himself in the film, he accepted the offer after just one phone call. Minnie Driver recorded the part of a patronizing grievance counselor in a scene that ended up being cut.Driver found recording her part "the funniest afternoon of her career",and was saddened when she was told it was cut.Isla Fisher and Erin Brockovich also recorded cameos, but they were cut as well.Kelsey Grammer recorded lines for Sideshow Bob, but he did not appear either.

Although he does not provide the voice, Arnold Schwarzenegger is President of the United States in the film. He was chosen ahead of the incumbent President George W. Bush because "in two years that'd mean the film [would be] out of date."This idea was met with nervousness, as Brooks noted "[Schwarzenegger's] opinion polls were way down, so we were sweating whether he'd make a political comeback, so it was great when he did."The animators began by drawing an accurate caricature of Schwarzenegger,but one of the staff suggested that an altered version of the recurring character Rainier Wolfcastle, himself a caricature of Schwarzenegger, could act as President.This idea was developed, with the design of Wolfcastle being edited to have more wrinkles under his eyes and a different hairstyle.