Paul Giamatti has just met the Prince of Wales. Not that Prince of Wales but the one known as David who would go on to become Edward VIII. And of course not the real David but an actor playing him, in full regalia at a party in Downton Abbey's UK Christmas special.
Sitting in a side room at Goldsmiths' Hall in London, which doubles as Buckingham Palace for the episode, trussed up in starched white tie and looking - as he often does - a little addled, Giamatti recounts the scene.
"I go blundering over to the Prince of Wales - thinking - I'll just shake his hand."
Needless to say the prince is a little taken aback at this flagrant breach of protocol, and merely looks through Giamatti's brash American businessman as if he were another footman.
"He blows me off. Hopefully it's funny. I do a lot of that in this episode."
Giamatti is the latest American guest star to arrive in Downtopia. Shirley MacLaine (who plays Lady Grantham's mother Martha Levinson) crossed the Atlantic for the opening two episodes of season three, and is back again for the Christmas special too.
But it's the casting of Giamatti that is quite the coup for Downton's producers. Giamatti is not merely famous but credible with it, ever since his brilliant lead performance in Alexander Payne's Sideways in 2004 brought him mainstream acclaim.
World-weary is his default position, he's a self-styled character actor who can inhabit leads, too, as he showed playing the second US president in HBO's 2008 miniseries John Adams. In the last year alone he's brought his wry intelligence and quicksilver presence to parts in everything from 12 Years a Slave to Saving Mr Banks.
In Downton Abbey, Giamatti plays Mrs Levinson's son and Cora's brother, Harold. The two visiting Americans form something of a Yankee double act in King George's court. The feature-length episode, which is rather uncharitably not set at Christmas, brings most of the cast to London for Lady Rose's (Lily James) presentation to the king in 1923. It includes a climactic debutante's ball at the Crawley's previously unseen London mansion.
"I'm this immensely wealthy businessman," Giamatti says. "Who had some sort of involvement in the crooked deals that were part of this thing called the Teapot Dome scandal during the President Harding administration. Harold has come to London to lay low."
Harold Levinson is roundly derided as a playboy by Cora and Robert. But in Giamatti's hands he's not particularly playboy-ish.
"I thought it was kind of interesting the way the character was written," Giamatti says. "He's awkward and not terribly comfortable in these social situations. He's more comfortable in business. He obviously whores around a lot on yachts and stuff like that but it's not his primary interest."
America's fascination with Downton Abbey is becoming almost as fascinating as the show itself. We have heard how everyone from P Diddy to Newt Gingrich wouldn't mind a spot on the show. But Paul Giamatti? Even he says he was taken back at the reaction to his casting.
"I have never done anything where it was announced I was going to do it and people went more crazy about it. I really haven't. I had perfect strangers coming up to me in the subway and the street."
Giamatti was born in 1967 in Connecticut, his father a Yale professor and his mother an English teacher at the prestigious Hopkins School. He is a Broadway veteran; I'm still not quite sure I believe that he is a devout Downton fan . . .
"I hadn't seen every episode of the show - now I've seen more of it - but I enjoyed it," he says, a little hazily.
And did the fact that he's a self-confessed Anglophile have anything to do with his decision to grab a couple of weeks in the UK?
"I love it over here. It's not the primary reason I did it but I was like . . . great, I get a free trip to London. The primary reason was the show. I just thought it would be fun to do something like this: the idea of being the American dropped down in the middle of it is funny to me."
I wonder if the appeal of Harold Levinson to Giamatti is because he is a strong, confident character, a man who, in spite of being snubbed by the Prince of Wales, is a winner. Giamatti plays crooks and cranks to perfection, with a particular sideline in hapless dupes.
"It's true - what is different about Harold is he's more straightforward powerful. He's not a mess, the way most of the guys I play are - a neurotic mess. I just did The Spider-Man movie (The Amazing Spider-Man 2) right before this where I played Rhino, this mobster who would scream and blow things up and shoot things. So it was nice to be a bit more normal. Get dressed up."