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How did Futurama have so many celebrity guest appearances?
(self.futurama)
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This is answer number 1. The guy from the Simpsons asks you do to voice work, which I have to imagine isn't as much of a pain in the ass as being filmed.
I think answer 1b is they are also getting paid for said work on a show by a legendary creator.
1c is they are usually themselves, which means there's no confusion as to who the person voicing the character is. It's great for their brand.