Over the years guest appearances have come thick and fast on The Simpsons. If you include the movie, there have been over 660 different celebrities on the show who between them have amassed a number of guest appearances not far shy of 1000. Some have been epic - like Albert Brooks aka Hank Scorpio - and some, simply put, haven't. Just ask Justin Bieber. There have also been stars who started as guests, but ended up fitting in so seamlessly or so successful that they were invited back regularly. Kelsey Grammar was so good as Sideshow Bob that he's become one of the most beloved guest characters the show has ever seen. Likewise Phil Hartman first appeared in a one off in 'Bart Gets Hit By A Car' in 1991 before going on to amass 52 guest appearances as Lionel Hutz and Troy McClure. But for all of those celebrities who became the foundation for some of the best Simpsons episodes, there were some who - rather unthinkably - actually turned down the chance to appear.