They can barely contain themselves.

We can barely contain ourselves. Grantham Villas – sorry, Downton Abbey – is to receive a special guest tomorrow night. Lady Grantham's mother is visiting from America and the cast are frankly a mess. For, as everybody knows, the new character Martha Levinson is being played by Shirley MacLaine, she of The Apartment, Sweet Charity, Being There, a genuine bona fide screen legend, a live flesh and blood link with a golden age of film. They just don't know what to do with themselves.

So many questions. How do they address her? "Your Hollywood-ship."? "Your Oscar-ness."? All through filming, they keep trying to be cool and professional, but so often, the stiff-upper-lipness required of their parts breaks down into good, old-fashioned gush.

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"Lady Levinson." (She's not really a Lady at all, but Carson is taking no chances.) "Tea is served in the drawing room if you would care to, oh God, can I just say it's so amazing to meet you. I remember watching The Apartment when I was still at school and I just absolutely loved it. So innocent and of its time and I love that scene where ..."

"CUT!" For the fifth time that morning, the director has to storm on to the set. "Please, I know you're just jobbing actors."

"Ha, impudent young man," sniffs Maggie Smith, still in character, fully imperious at the back of the drawing room and she harrumphs off the set, striking the young man with the clapperboard to the ground as she does so.

"But please, can we all try and get a grip. Thank you. And - action."

Yet on this particular day something special happens.

Martha/Shirley drops her fur stole, turns coquettishly to Carson, runs one finger under his chin, causing him to drop the tea tray which no-one notices because they're all transfixed as Shirley – for this is most definitely Shirley MacLaine now – hitches up her skirt, steps on to the chaise longue and then on to the table where she sweeps the silver fruit bowl and candelabra to the floor, turns to the camera and begins to slowly sing: "If – they – could – see me now/That little gang of mine/In this fancy house/Drinking this fancy wine -"

Every mouth is open. Even the director is silenced. Then there is a series of thumps as the entire starstruck cast falls to the ground.