The official site's Homing Beacon newsletter features a portion of a transcript of a video interview with Ian McDiarmid. Here's an excerpt:

"I heard that George was indeed going to make those prequels, and that he was coming to London and he wanted to see me. I had no idea, I suppose, that the Emperor in some form would be in it." After a brief meeting with Lucas (10 minutes in length, McDiarmid recalls) it became clear that Palpatine was not only returning, but that he would be central to the events in this new trilogy.

"I was sort of reeling," McDiarmid says. "Then he explained to me, not in great detail, about the trajectory of the three movies, and my character in relation to that. But he talked about two people. He talked about this perfectly ordinary, rather charming, politician who one day became Emperor. And that story would be seen in films 1 and 2, and in film 3 he would move closer to the center as he got closer to power."

Unlike the face-obscuring role of the Emperor, this time McDiarmid would keep his own features for the role of Palpatine. "This only happened because I was the right age," says McDiarmid. "I was the right age, ironically, for the first prequel when it was made. I wouldn't have been if he'd started with [Episode I]. So I was in the very strange and rather wonderful paradox of playing myself when young at my own age, having played myself previously when 100-and-I-don't-know-what. So that was, of course, extremely interesting and a challenge, and it was wonderful to think that I'd be part of the continuing saga. And of course I went home and refused to believe it. But bit by bit it became a reality."

The newsletter adds that the full interview will be screened at Celebration III and will also be available to Hyperspace subscribers.