The Downton Abbey actor joins Sheridan Smith, Johnny Vegas and Pudsey the dog in BBC’s festive adaptation of David Walliams’ children’s story

The Hugh Bonneville sitting opposite me in BBC’s Broadcasting House bears close resemblance to his Downton Abbey alter-ego Lord Grantham – a role he has played to great acclaim from both sides of the Atlantic. But it wasn’t so long ago that he was sporting a scraggy grey beard and ill-fitted trench coat to play David Walliams’ titular character Mr Stink in the BBC’s festive 3D adaptation.

He recalls a text from the head of BBC comedy saying, “Don’t shave your beard off – do you want to play Mr Stink?” And it’s not the first time his “annual treat” of growing facial hair has come up trumps (he played a bearded Captain Henry Avery in Doctor Who episodes The Curse of the Black Spot and A Good Man Goes to War).

But this particular role is far removed from his position as head of the Crawley family. “Playing a very messy, curmudgeonly tramp was a lovely departure,” he explains, “mainly because it meant I didn’t have to take all my costume off at lunchtime for fear of smearing it with my spaghetti Bolognese. In fact, it was encouraged that I eat my lunch in costume!”

For those uninitiated in Walliams’ popular children’s tale – illustrated by Roald Dahl’s long-time collaborator Quentin Blake – it follows 12-year-old Chloe, a lonely outsider who’s teased at school and finds solace in her friendship with mysterious, smelly tramp, Mr Stink. “He has – in our case – a visible wall of aroma around him that keeps people away and [the BBC] are going to be doing some CGI to have this stink, which in the book is very visual, following me around.

“Chloe has an over-achieving younger sister, a very pushy mum and a dad who’s trying to keep up with the rest of the family so she finds solace in a man who is outside the world around him and gradually they form a bond. He makes a little change in their family life and she makes a very big change in his.”

The Christmas family comedy boasts an impressive cast that includes Sheridan Smith, Johnny Vegas and Pudsey the dog – plus a cameo from Walliams himself. “He’s in our faces for most of the year in some way or other, and in this case he’s in my face as the Prime Minister.

“It must be very nerve-wracking when you’ve slaved away in your own office creating a story and then to have it wrestled out of your hands by third parties who can muck it up on screen.

“It’s great that he’s in it because he’s one of our funniest people so to have him and Pudsey in the same frame was great.”

Bonneville is no stranger to acting alongside four-legged friends, sharing much of his time on-screen in Downton with his character’s beloved dog, Isis. But he won’t be drawn on which pooch is better behaved… “I couldn’t possibly comment about my leading ladies, but they both love treats. That is their guiding principle in life – as long as their trailer has treats they’ll do anything. A bit like actors, really!”

In addition to its experienced cast, Mr Stink sees the acting debut of Nell Tiger Free who plays Chloe alongside Bonneville and he is quick to sing the praises of his young co-star: “she wasn’t nervous but she was very conscious that she wanted to get it right and she really, really did.

“She’s got a great spirit about her. On the final day she got all her supporting artists to do her Gangnam Style dance and had them all jigging around the aisles.” So will we be seeing Lord Grantham recreating Psy’s cowboy jig? “Not if I can get hold of her YouTube copy first!”

With such a rich ensemble cast, it sounds like there was no end to on-set hilarity when filming took place this October. “There was a lot of giggling, largely based on Sheridan Smith’s beehive. She has an extraordinary hair-do that grows and collapses with her fortunes. It was great – we had a lot of laughs. She was playing Mrs Crumb by day and Hedda Gabler at night – her range is astonishing.”

But despite the comedic aspects of Walliams’ plot, Bonneville is quick to point out the serious undertones to the story, explicitly stated by Mr Stink himself. “He says if there were more girls like Chloe, perhaps there wouldn’t be as many homeless people in the world. And I suppose that’s a simple thing – certainly I don’t do enough. I don’t go out there every night to soup kitchens to help. I try to do my bit for charity but I’m as guilty as anyone of not doing everything I possibly can to make sure there isn’t homelessness.

“There are a million reasons why people are homeless and none of them good and this show isn’t saying we should all give up our Christmas turkeys and get out and “hug a homeless” as David Walliams’ character suggests, but if it helps one person think, ‘I would actually like to do a bit more’ then great.”

So what can viewers expect if they tune in this Christmas? “Little moments of charming, contemporary comedy about the way we live now, but at the same time it’s a traditional story in terms of redemption and forgiveness and misunderstandings of the way we are.” Sounds like perfect family festive viewing to us…