When not terrorising the universe, Scots-born McDiarmid spends much of his time working in the theatre and from 1990 to 2002 he was the Joint Artistic Director of the Almeida Theatre in London. When he hasn't been treading the boards, he's appeared in Gorky Park, Dirty Rotten Scoundrels and Sleepy Hollow. But it's as the biggest baddie of them all - the Emperor in the Star Wars franchise - that film fans will remember him for. He has played Palpatine in Star Wars Episodes l, ll, lll and VI.
How do you approach so dark and Satanic a character as Chancellor Palpatine?
Well, for those who didn't see Return Of The Jedi, I just played a straightforward politician - charming, smiling, out for the good of the universe. But underneath that there lurks a monster. So it was very easy to build the character - I just looked in the newspapers.
Did the make up you wore during Palpatine's transformation to Darth Sidious help?
It always helps. It certainly helps to be a monster in monstrous make up. But George was very interesting when we started The Phantom Menace: he told me I should think of my eyes as Palpatine's contact lenses, which was a great thing to say to an actor. So my face was actually Sidious' mask, and then when I put on the mask I became him. That kind of schizophrenia is always fun to play, and in this film it's great because one explodes through the other. In this film he's worse than the Devil and certainly worse than Darth Vader, whom I think comes across as more sympathetic than people might imagine.
What's the best part of playing a character who is the darkest of the dark?
He is the blackest of the black. It's fun, you're not going to get a part like that every day. I like the fact that he doesn't really have any psychological workings. He was spawned in Hell in a way, as Siths are apparently. They can't get better. Except as you see in the movie he goes to the opera. So his one redeemable feature is that he's a patron of the arts.
Star Wars: Episode III - Revenge Of The Sith is released in UK cinemas on Thursday 19th May 2005.