During the 40th-anniversary panel at SWC Orlando, Ian McDiarmid, the actor who plays the Emperor, discussed his role. During the Q&A McDiarmid was asked how George Lucas prepared him to play the villainous character, to which he replied: “Well, he didn’t.”
Apparently, Lucas isn’t much for explanation so McDiarmid learned most of the information from the script later on.
“George doesn’t go in for lengthy explanations, which is great. We met very briefly over lunchtime, with [director] Richard Marquand. This is, of course, Return of the Jedi. I was very pleased to meet them both, thrilled, of course.”
“And then we said our polite goodbyes and I went back home and my phone was ringing. My agent said ‘You’ve got the part.’ I said, ‘That’s great. What’s the part?’ He went leafing through piles of papers and he said, ‘Well, apparently, he’s called the Emperor of the Universe,’ and I said, ‘Well, I guess we’re doing it, then.'”
“From there, it was makeup, hood, yellow eyes, and a script. I think most of my information, and most actors would agree about this, came from the script. George was always around and very helpful, but he wasn’t specifically directing that movie, that was Richard.”
By the time McDiarmid was playing the Emperor in the prequels, he had a far better grasp on the villain.