I play the Lord of the Mings

Hugh Bonneville goes from Downton to down-and-out for BBC drama Mr Stink. The star tells James Rampton why it's not a role to be sniffed at

TV critics don't come much more powerful than this: Michelle Obama loves Downton Abbey. In fact, the First Lady is such a huge fan that her staff have apparently been begging ITV for advance copies of the third series, which does not go out in the US until next month. Her husband is also reportedly a massive aficionado of Julian Fellowes's period drama.

For all that, Hugh Bonneville was astounded when the presidential passion for the programme translated into an invitation to dinner at the White House in March. "It was rather strange – in a good way!" smiles the actor best known for playing Lord Grantham in the hit costume drama.

"To go to the White House is a once-in-a-lifetime opportunity. In fact, the building is not that big. It's just a modest 20-up, 20-down!

"But it was a really special day, and I felt very welcomed. At moments like that, you do think, 'My goodness me, this show that was very hard to make in a soggy field near Newbury has been grasped to people's bosom in so many countries'. You do have to pinch yourself."

For his latest project, Bonneville finds himself in rather less elevated surroundings. In Mr Stink, which goes out on BBC1 on Sunday, he gets down and dirty as the titular tramp who is more likely to be found in the workhouse than the White House.

In this charming adaptation of David Walliams's bestselling children's novel, the big BBC family drama this Christmas, Mr Stink is a pongy hobo who has chosen to cut himself off from mainstream society.

However, he is gradually reintegrated after he is befriended on his park bench by a young girl called Chloe (Nell Tiger Free). Taking pity on a fellow outsider, she invites him to live secretly in her family's garden shed.

We are on the set of Mr Stink, which has been adapted for the screen by Walliams and Simon Nye (Men Behaving Badly). The location suggests that there is a corner of west Ruislip that is forever Lapland. For the climactic scenes of the drama, a close in this west London suburb has been transformed into a winter wonderland, complete with full snow covering, inflatable snowmen and dancing neon Santas.

Bonneville is the opposite of Mr Stink. Where the tramp is curmudgeonly and crotchety, the actor is warm and witty. He also does a very nice line in self-deprecating humour. At one point he grins and says that, "Frankly, all the parts I play are the same performance, just different outfits!"

Later, Bonneville jokes that, "If people say this drama is not as good as the book, you can absolutely blame the author as he has done the adaptation himself!"

He goes on to reflect on the fact that many high-up people in the Obama administration are enormous fans of Homeland. "Are the figures turning the wheels of power in the US sending out to their local shop for the next box set, rather than sorting out their Middle East policy?"

Today, the actor is dressed as his whiffy character – in a huge tramp's coat tied up with string, a matted beard and a hairdo which looks thick and tangled enough for birds to nest in – and advises me not to come too close.

Pointing to his filthy feet, Bonneville laughs and says: "The full dirty, snot-encrusted make-up takes one and three quarter hours to complete. They're very thorough about it. When I finish filming, I get back home and think, 'What is the point of taking it off? I have got to put it on again in a minute!'"

The actor says he was drawn to this enchanting, if malodorous fairytale because, "it has a Roald Dahlesque relish of stinks that all boys can enjoy well into their fifties. But it also has a deeply touching heart. It's a story that young people can enjoy with a very simple message: be nice to people less fortunate than yourself."

Bonneville thinks that Mr Stink also scores because "it gives great credibility to young people".

He adds: "Young people are so often dissed by the media. David has this wonderful childlike – not to say infantile! – attitude. In this book, he is presenting a child's view of the world. He realises they get a bad press and wants to change that.

"Mr Stink's relationship with Chloe is very touching. Like him, she feels an outsider in life because she is bullied at school. She is Every Girl. Everyone at some point in their lives feels excluded and misunderstood. But she and Mr Stink find a kinship. She's far more active at reaching out and making a connection, and that gives him hope about young people."

Previously, Mr Stink's strongest relationship had been with his dog, the Duchess, who in the drama is played by Pudsey, better known as the winner of this year's Britain's Got Talent competition.

Bonneville says: "Having spent the last three years acting with a labrador on Downton Abbey, I'm used to the dog being the biggest star of the show. Pudsey is in fact the biggest star in Britain. Talk about Hollywood behaviour! I'm not allowed to look him in the eye before noon and I have to call him 'Mr Pudsey'.

"He has so many engagements, but he deigns to visit us for a few hours every now and again.

"I thought Julia Roberts had a big trailer on Notting Hill, until I saw Pudsey's."

Bonneville will be starring in the seasonal special of Downton Abbey on Christmas Day. He is still recovering from the storm of criticism his character received for ignoring medical advice and thereby contributing to the death of his beloved daughter Sybil (Jessica Brown Findlay.)

He says: "People on Twitter were devastated. I've had hate mail and bomb threats, because I'm obviously the most evil person on the planet for favouring one piece of medical advice over another.

"I don't want you to take this the wrong way, but it's lovely that people are devastated about Sybil's death. It shows they've invested in the world, and it's a real compliment that they care so much."

'Mr Stink' is on Sunday at 6.30pm on BBC1