William Shatner holds a special place in the lore of The Simpsons despite never appearing on the show””rumor has it he was the very first celebrity to turn them down. Not much is known about the part he was offered, aside from the fact it must have come pretty early in the show's history. Maybe Shatner would reconsider now that the show is a huge success, but it would appear they haven't tried asking him again.
Prince also holds a special place in Simpsons history, in that the plot of the episode he would have appeared in is well known””because the writers decided they couldn't make it without him. Unfortunately, Prince decided they couldn't make it at all, in fact, so he rewrote the entire episode himself to be more befitting of His Royal Badness. Communications broke down sometime after the Simpsons writers realized Prince didn't seem to have the best grasp on reality...uh, Springfield's reality.
The Boss has been approached by the show at least three times, and each time he had to be replaced with a pretty big name. First he was to appear in "One Fish, Two Fish, Blowfish, Blue Fish," only to be replaced by Larry King. Next, Sting took his role in "Radio Bart." Finally, he turned down "When You Dish Upon a Star." There isn't much explanation why, aside from the fact he is always touring and doesn't often make guest appearances in general.
Another regularly requested, always denied performer, Dylan first turned down a chance to headline "Homerpalooza," and was thus replaced by Peter Frampton. I'm sure concert-goers in Springfield still get sad when they think back to that terrible concert year. Later he was offered a role in "El Viaje Misterioso de Nuestro Jomer," which went to Johnny Cash when he turned it down. No complaints about that one.
Like with Dylan leading to Cash, sometimes a major guest star dropping out is a blessing in disguise. Michael Caine was the first choice for James Woods' role in "Homer and Apu," but Woods would go on to be one of the favorite guest stars of The Simpsons animation staff””and many of the fans.
More commonly, a huge star saying no meant a Simpsons regular got an extra job. Cruise said no to a role "Brother From the Same Planet," which then went to Phil Hartman. Hartman knocked it out of the park as always, but it would have been interesting to have heard the man who clearly inspired the role give it a shot.
It doesn't always work out so smoothly””for "Last Exit to Springfield," the role of evil dentist Dr. Wolfe was offered to both Clint Eastwood and Anthony Hopkins, and both said no. Anthony Perkins was also approached, and wanted to accept it, but unfortunately died before he could take the role. Cast member Hank Azaria finally took the role in the end.
One might think pop culture impresario Quentin Tarantino would have been overjoyed at a chance to appear on The Simpsons, but sometimes the writers bite a bit too hard. Tarantino was asked to play himself in "Simpsoncalifragilisticexpiala(Annoyed Grunt)cious," but objected to his portrayal, particularly his lines. Dan Castellaneta, less concerned with Tarantino's image, voiced his character instead.
In what would have been a case of two cult and pop icons merging for something likely absurd but likely hilarious, Frank Zappa actually wished to appear on the show, and of course the producers wanted him. But unfortunately, timing is everything, and The Simpsons was rising near the end of Zappa's life, and he was too ill to ever actually make an appearance. Only a "No" on technicality, but it still could have been.
Another cultural icon in his own right, another technical "no thank you"””Carrey was set to voice a singing hobo on "Simpsons Tall Tales," but 2001 was a pretty busy year for Jim Carrey, and he was never able to do it. The omni-talented Simpsons cast stepped in as always, with Hank Azaria taking the role as he often did.