He certainly won't have slept much last night. He has already told her to prepare a speech and he's placing a bet. He thinks her dazed and confused Queen Victoria is the best thing she has ever done in the cinema. "She's never managed to carry off that luminosity before that she has in the theatre," he says. "Yes, I had a weep." He pauses and smiles. "She makes me weep a lot, living with her. No, seriously, she makes me laugh a lot as well."
Underneath this modest exterior lurks a pocket battleship. Williams earned his stripes over 14 years with the Royal Shakespeare Company under the guidance of Peter Hall, and he easily held his own by Dame Judi's side in the West End treats Pack of Lies and Mr and Mrs Nobody.
On television the duo were something of a staple diet in the early Eighties, as Mike and Laura in Bob Larbey's A Fine Romance and he surprised sceptics with his jaded music hall comic in the ITV saga September Song. "I never looked on acting as an ego trip," he says. "I could sight-read Dickens better than anybody else in class. I couldn't do anything else. I was a total dunce, apart from a bit of cricket and rugby. I was useless. But this one thing, I could do."
And so to Brief Lives, based on a book of short biographies written by a Wiltshire gentleman, John Aubrey, in the late 17th-century but never published in his lifetime.
This is tour de force stuff, some 56 pages of script in slightly over two hours under Garland's direction. Williams and Dame Judi have been married for 27 years and remain blissfully happy, although there have been traumas. Five years ago, fire destroyed their Grade II listed house in Hampstead.
Their daughter Finty (real name Tar Cressida) is an actress and mother to Sam who is now nine-months-old. As a former president of the Roman Catholic Actors Guild, Williams was hardly amused when his unmarried daughter announced the impending birth. And he was more than tetchy when she rolled around in the buff with actor Neil Pearson in the David Hare film Secret Rapture. He's old-fashioned and doesn't mind saying so. He was never a hell-raiser, though he did consider changing his name to O'Leary' to make him sound more "dangerous".
"I can't pretend it wasn't a shock when we first heard about the baby," he says. "We both would have liked it to have been done in a different way. But Fint is no longer a young girl. It was her decision and when we were confronted, we gave her all the support we could. It's true that I was angry when she once did a love scene in a film and didn't tell me. I didn't approve. There have been things I have turned down because the content has disturbed me. There is a line I won't cross."
He has been put to the test. He giggles, schoolboy-like, over his role as a lecherous French aristocrat on the prowl in the Seventies television production Elizabeth R, with Glenda Jackson. The director made him browse through a selection of photos of nubile models and asked him to pick which girl he'd like to sleep with.
"I was so embarrassed," he says. "Eventually I chose this Swedish girl. It was a non-speaking part. She just had to lay there. When she arrived she said she was very worried about being naked and would I meet her boyfriend who was just as concerned. Suddenly, there he was, all seven feet of him. I gulped and said instantly. 'If it's any consolation my wife is even more distraught than you!'
"But if you've got the best at home why go out searching for second-best? It's interesting that when we did A Fine Romance we took hold of the public's imagination as a couple seeking happiness. I understand we influenced a lot of people within the profession who generally regarded marriage and two people staying together for any length of time as something virtually impossible."
Williams, now 62, is still oddly youthful with his perennial Just William hair-do, now with a dash of grey at the sides. He and Dame Judi have done it all. They are, to their peers, the lovable luvvies. And though her awards went up in flames back in 1993 she may well soon have a new prize to place on the mantelpiece.
Can Finty follow in those footsteps? "Being the daughter of Judi Dench, people expect her to be a cut above the rest," he says. "That kind of pressure doesn't give her the chance she might have in other circumstances. One old friend once described the ability to act as 'the trick'. Jude had it. Fint has it. She has that same spirit and power. Yes, she's a chip off the old block. But we shall just have to wait and see."
'Brief Lives' opens at the Duchess Theatre tonight.