20. Martin Sheen
Character: The 'real' Principal Skinner
Episode: 'The Principal And The Pauper', Season 9 ('97-'98)
According to producer Josh Weinstein, Sheen was chosen for the role of rigid military man and Vietnam vet Seymour Skinner because of his work as a scuzzy renegade-hunter in Apocalypse Now.
The actor aced his part as the prodigal disciplinarian, but the episode itself fared less well – it’s often held up as the first sign that the show's golden age was coming to an end.
19. James Woods
Character: Himself, researching a role at the Kwik-e Mart
Episode: 'Homer And Apu', Season 5 ('93-'94)
Woods was on a shortlist of actors that producer David Mirkin was desperate to feature on the show. When first-choice Michael Caine turned down a chance to appear as an actor preparing for an upcoming role, the script was rewritten for JW.
On top of his excellent ad-lib skills, the crew remember Woods for his habit of artful hesitating (all part of his regular anxious-menace schtick) which made his animated character seem more realistic.
18. Kirk Douglas
Character: Chester J. Jampwick, dispossessed Itchy & Scratchy creator
Episode: 'The Day The Violence Died', Season 7 ('95-'96)
Douglas wasn’t the first choice for grizzled down-and-out – and Itchy & Scratchy creator – Chester J Lampwick. That was Prizzi's Honour star William Hickey, who the producers described as "famous for being a grouchy, mean old man".
But Douglas could hardly have been grouchier. He refused to wear the headphones during recording as they hurt his ears (meaning he couldn’t hear direction from outside the booth) and refused to do more than two takes per line.