The voice cast was signed on to do the film in 2001, and work then began on the script. The producers were initially worried that creating a film would have a negative effect on the series, as they did not have enough crew to focus their attention on both projects. As the series progressed, additional writers and animators were hired so that both the show and the film could be produced at the same time. Groening and James L. Brooks invited back Mike Scully and Al Jean (who continued to work as showrunner on the television series) to produce the film with them. They then signed David Silverman (who, in anticipation of the project, had quit his job at Pixar) to direct the film. The "strongest possible" writing team was assembled, with many of the writers from the show's early seasons being chosen. David Mirkin, Mike Reiss, George Meyer, John Swartzwelder, and Jon Vitti were selected. Ian Maxtone-Graham and Matt Selman joined later, and Brooks, Groening, Scully, and Jean also wrote parts of the script. Sam Simon did not return having left the show over creative differences in 1993. Former writer Conan O'Brien expressed interest in working with the "Simpsons" staff again, although he later joked that "I worry that the "Simpsons"-writing portion of my brain has been destroyed after 14 years of talking to Lindsay Lohan and that guy from "One Tree Hill", so maybe it's all for the best." The same went for director Brad Bird who said he had "entertained fantasies of asking if could work on the movie", but did not have enough time due to work on "Ratatouille". The producers arranged a deal with Fox that would allow them to abandon production of the film at any point if they felt the script was unsatisfactory.