There are several empty plates in front of him, along with a half-drunk glass of wine. The 53-year-old actor is in high spirits – and deservedly so.
This year sees him star in two new BBC shows – as the butler in period comedy Blandings and in the title role for detective series Father Brown.
Williams, who rose to fame in BBC comedy The Fast Show, is now best known around the world as Arthur Weasley in the Harry Potter films, the father of Harry's best friend Ron.
Since the last of the seven films, released in 2011, Williams has enjoyed keeping himself busy with a variety of roles, including two appearances in Doctor Who last year.
"Harry Potter sometimes got difficult because I couldn't do any-thing else. So when it finished I was like a cork coming out of a pop gun and had a great time being asked to play baddies and lots of things," he says.
"I did a George Gently, and The Indian Doctor and Hustle, and I've done Dr Who, Blandings and Father Brown, so it's been good."
Based on the short stories by GK Chesterton, Father Brown is an amicable Catholic priest who just happens to have a sideline in solving crimes.
"I was working on Blandings and my agent phoned and said the Father Brown production had made an offer.
"I said, 'What part?', thinking it would be the gardener or a baddie, and she said, 'Father Brown'. "I remember the pause where I went 'OK... I'll have that'," he adds in his Midlands accent, chuckling.
Becoming a man of the cloth didn't faze Williams in the slightest ("I've played a few vicars before, so I've been there").
Known for being a character actor, this is one of the first times Williams has taken on the lead.
He says: "Most character is gen-erated by the way people treat you, not by what you do.
"That is why often it's easier to be the central character because you're the centre of attention and people generate your status.
"And it's quite easy with Father Brown because he has genuine humility so you haven't got to create an inflated character."
Like Agatha Christie's famous sleuths Miss Marple and Hercule Poirot, friendly and forgiving Father Brown is an unlikely detective.
"I think that's a sort of English trope, the unlikely anything," says Williams.
"The 'whodunnits' are sort of like cowboy films in America. They have a tone of their own, a landscape and a set of characters and images. It's a milieu. In itself it's a stage set, so that's why I think it translates so well to television – it's like Cluedo isn't it?"
Williams also plays Beach the butler in the new BBC One adaptation of PG Woodhouse's Blandings Castle stories, alongside Timothy Spall, Jennifer Saunders and David Walliams.
The series revolves around the hilarious misadventures of Lord Emsworth, his widowed sister Lady Keeble and the guests and staff of Blandings Castle.
THESE often involve Lord Emsworth's beloved prize pig The Empress of Blandings.
As long-suffering Beach, often charged to help with the care of The Empress, Williams had to work with a real pig during filming.
"It's a Middle White, not a Gloucester Old Spot [as in the books] so there will be questions asked," he quips.
After so long working on the Harry Potter films, the cast have often said they became like a family. Williams insists that while it was sad to say goodbye to the role, there was an overwhelming sense of achievement.
"It was race run. We made eight films with pretty much the same cast and crew, so it's like, 'Right we've done that.' That was a sense of completion, not of truncation."
Happy to discuss Harry Potter, Williams becomes stony-faced when asked about his time on hit Nineties sketch show The Fast Show, in which he starred with Harry Enfield and Paul Whitehouse.
Famous for its catchphrases – "Suits you sir" – and starry cameos, surely Williams must share fans' fond memories?
"Kind of..." he says, wincing. "But it's a double-edged sword for me, because people for a while seemed to assume I was a comedian, which I've never been," he says.
"I've never done stand-up, I came via small-scale touring theatre, through the Royal Shakespeare Company, the National Theatre, then I got employed on that as an actor who had a humorous sensibility."
After an awkward pause, then he suddenly continues: "And I'm also getting really bored of being 'Mark Williams – The Fast Show, Harry Potter!"'
"I've done about 15 big films. In America, I'm a British film character actor. Here, it's a bit weird really. People don't differentiate. It's like being a bass player and people calling you a drummer all the time. After a while, it's like, 'Oh shut up!"'
The prickliness quickly fades and he's back to being jovial and warm. Being a diva doesn't suit Mark Williams.
l Blandings begins on BBC One on Sunday; Father Brown begins on BBC One on Monday