Downton Abbey’s Dan Stevens goes to Hollywood

To get the look, he had, he explains, to eat “a lot of protein very regularly”; to “feed the fire”. Then he looks a trifle embarrassed. “There are all these phrases that they throw at you . . .”

Stevens, 31, is known for his exquisite manners (“You’re not supposed to die on Christmas Day on British TV,” he has apologised). Forget Hollywood behaviour, when he wants a coffee, he asks as if he’d perfectly understand if this were too much of a bother then, when it’s brought, apologises to the person bringing it, then thanks them; then thanks them again for good measure. But there’s no denying his new status. Since making his Broadway debut in New York last year, starring opposite Oscar nominee Jessica Chastain in The Heiress, based on a Henry James novella, Stevens has moved out to the city permanently with his wife, Susie Hariet, and their two small children.

In his Downton days, an interviewer once described him as looking a bit “pasty”. But as with so many English actors who move across the Atlantic, America serves as a great optician whose intense lens brings the fuzzy corners of a human into focus — squaring jaws, toning skin, sharpening lines.

He’s lost about 30lb and, rumour has it, was made to give up dairy. He calls this “hilarious”, not looking, it must be said, all that tickled. “I don’t know what people think. It’s like it’s a [cliché] of Hollywood types coming into a room and saying, ‘Lose 2st, kid’. I’m not being bullied by anyone.

“Dieting is dangerous,” he continues, “because you’re putting yourself in a mental place of deprivation. If you want to have a cream cake, have a cream cake. It’s more of an attitude.” Though, for the record, that attitude does involve being largely off dairy, “and the gluten free thing is very big in America”. So not that many cream cakes.

The move to the US happened after that Christmas finale of Downton in which Stevens was dispatched. He’d told them between seasons two and three that he wanted to leave, “to give them time to plot Matthew’s doom”. And what a doom it was: an overturned car, with the Crawley heir and master lying with blood trickling from his ear. Was that writer Julian Fellowes’s revenge on him for leaving? “No the blood was my idea. We wanted to make sure that it was clear what had happened.”

Fellowes was not miffed that he left, Stevens says, briskly. “I mean Julian’s an actor and he understands that trajectory better than a lot of people involved in that level in Downton.”

Stevens already had a promising career. Straight out of Cambridge, he first came to attention in 2006 in the BBC’s adaptation of Alan Hollinghurst’s The Line of Beauty, where he was brilliant as the young gay social climber who gets to dance with Margaret Thatcher. He followed it up with Sense & Sensibility (he played Edward Ferrars) before joining Downton.

One suspects he may have tired a little of the bizarre plot twists. In series two, Stevens was required to play a man who had been paralysed and rendered impotent by his role in the First World War. Then, suddenly, there was an episode where he rose, as critics said, Lazarus-like from his wheelchair, exclaiming that he could “feel a tingling”. In his legs, everyone hastened to add later; though Twitter smirked nevertheless.

So was he fed up? “I . . . ” Pause. “No . . .” Pause “I just . . . it was the first time I’d done a long-running format like that, so after three years, filming for six, seven months a year. I was just in the mood for a change. As simple as that.” When he left he didn’t have another job. Was he scared? “Completely. It was sort of thrillingly terrifying.”

He upped sticks and went to New York with Hariet, a jazz singer whom he’d met while she was in a musical and he was acting in Romans in Britain, a play that also famously had a lot of nudity. “Hers was a cast of lovely young girls and ours was a cast of lovely young boys and we hung out a lot. She came and saw Romans in Britain a few times,” he says, giving a laugh that you could only describe as a trifle dirty.

They soon married and their children, Willow and Aubrey, are now 3 and 2. Was the move to New York a risk for such a young family?

“That was a big moment for my wife and I, she was pregnant with our second child,” he says. “[She was] really up for the adventure. My wife is really supportive. It’s not a decision you can make on your own; you can’t say, ‘Darling we’re moving, pack your bags.’ That’s not how it works.”

He’s always loved New York (“it has a buzz to it; it really fires me up”). Willow has started school. “It’s been a baptism of fire becoming acquainted with the New York School system, for sure. You hear people talking about preschool the way that people talk about universities here. But she’s 3. That to me was quite shocking.” Is it, I wonder, as bad as London nurseries that charge £15,000 a year? He laughs. “Is that all it is over here?

Stevens has spoken fondly of his own childhood. He was adopted when he was a week old, and raised in Essex and Wales. His parents, retired teachers, live in Sussex. He has a younger brother, also adopted. At the age of 10, he was sent to boarding school at Tonbridge in Kent, whose fellow pupils he once described as “almost wilfully stupid and ignorant”, but today he calls the school “a very vibrant place”. You get the feeling that, whatever shape he’s in now, he was definitely a bookish teen rather than a jock.

“I guess I had thought that the two things were mutually exclusive. That’s partly ingrained in the education system; if you’re at all brainy and nerdy you should stay towards the science block and apply to Oxford and Cambridge, and if you’re sporty then you can go and do your sport thing. That to me is mad. It’s been a real delight to reconnect with a more physical side of things.”

He found institutionalised discipline tough. He smoked at school because it was illegal; at university, where it was allowed, he gave up. But he does admit that places such as Tonbridge give you an advantage. “That confidence can serve you very well; to go into social situations and feel that you can tackle things head on.” Though he is also aware that it can become too much. “It can spill over into a sense of entitlement that I don’t find at all appealing.”

At Cambridge he read English and became friends with Rebecca Hall, Tom Hiddleston and Eddie Redmayne. Peter Hall cast him in a touring production of As You Like It, after seeing him in a play with Rebecca. Were they aware that they were going to be the next glittering youth?

“That would have been a little precocious. When you’re there that’s just what it’s like. It was a fun time.” He and Hall were flatmates for a few years when they moved to London after university. They are “still great friends. She’s godmother to my daughter.”

There does seem to have been something a bit precocious about Stevens. The late novelist and poetry evangelist Josephine Hart hosted live Poetry Hour readings with him: then he began making guest appearances (as the thinking-person’s actor) on BBC Two’s Friday night arts round-up, The Review Show. One evening he criticised the timidity or rather “readability” of Booker shortlists on the show; a few weeks later Peter Stothard , chair of the 2012 Booker prize, phoned him up asking him to be a judge. Hilary Mantel won that year.

Now he has several films out this autumn, including The Cobbler, a comedy with Adam Sandler, Steve Buscemi and Dustin Hoffman, and A Walk Among The Tombstones, playing a Nabokov-reading, fine art-collecting New York drug dealer opposite Liam Neeson’s retired cop. He also takes a turn as Sir Lancelot opposite Ben Stiller in Night at the Museum 3. Stiller, apparently, is a huge Downton fan. On set, the two of them used to play a multiple-choice quiz called QuizUp on Downton Abbey. Stiller always won.

But you sense it’s The Guest that signals a real departure. He says he was completely won over by it. “I’ve never laughed as much reading a script.” Though you wonder what an ex-Booker judge makes of all the cartoon violence — this is, after all, the sort of film where people get stabbed and shot and then stand up again.

“Oh, watch it again,” he says, jovially, like a tutor telling you to reread Beckett. “You missed something. I think this is a very clever film.” But, I persist, didn’t he find the script just a little silly in places?

“No sillier than having to get out of a wheelchair.” It’s a fair point.

The Guest is out on general release