prolific writer on The Simpsons. One of the best-known former writers is Conan O'Brien, who contributed to several episodes in the early 1990s before replacing David Letterman as host of the talk show Late Night. English comedian Ricky Gervais wrote the episode "Homer Simpson, This Is Your Wife", becoming the first celebrity to both write and guest star in the same episode. Seth Rogen and Evan Goldberg, writers of the film Superbad, wrote the episode "Homer the Whopper", with Rogen voicing a character in it. At the end of 2007, the writers of The Simpsons went on strike together with the other members of the Writers Guild of America, East. The show's writers had joined the guild in 1998. Voice actors The Simpsons has six main cast members: Dan Castellaneta, Julie Kavner, Nancy Cartwright, Yeardley Smith, Hank Azaria, and Harry Shearer. Castellaneta voices Homer Simpson, Grampa Simpson, Krusty the Clown, Groundskeeper Willie, Mayor Quimby, Barney Gumble, and other adult, male characters. Julie Kavner voices Marge Simpson and Patty and Selma, as well as several minor characters. Castellaneta and Kavner had been a part of The Tracey Ullman Show cast and were given the parts so that new actors would not be needed. Cartwright voices Bart Simpson, Nelson Muntz, Ralph Wiggum and other children. Smith, the voice of Lisa Simpson, is the only cast member who regularly voices only one character, although she occasionally plays other episodic characters. The producers decided to hold casting for the roles of Bart and Lisa. Smith had initially been asked to audition for the role of Bart, but casting director Bonita Pietila believed her voice was too high, so she was given the role of Lisa instead. Cartwright was originally brought in to voice Lisa, but upon arriving at the audition, she found that Lisa was simply described as the "middle child" and at the time did not have much personality. Cartwright became more interested in the role of Bart, who was described as "devious, underachieving, school-hating, irreverent, [and] clever". Groening let her try out for the part instead, and upon hearing her read, gave her the job on the spot. Cartwright is the only one of the six main Simpsons cast members who had been professionally trained in voice acting prior to working on the show. Azaria and Shearer do not voice members of the title family, but play a majority of the male