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Zero Mostel (1915-1977) was an American comedian, actor and singer. He passed away September 8, 1977 before his Muppet Show episode aired (Episode 202). He provided the voice of the evil Spellbinder on The Electric Company.
(Commenting on Jim Henson's Muppets): "He has the best possible actors. If you have a disagreement with them, you can always use them to wash your car."