Dan Stevens after Downton Abbey: The erstwhile Matthew Crawley is back in period costume

He left fans bereft with his departure from the nation's favourite posh soap. Now he's in the movies – and, he tells Matt Mueller, his appearance in Summer in February is a labour of love

The pavement in front of the Athenaeum gentlemen's club in Pall Mall is scattered with dirt and gravel, a horse and carriage is parked at the entrance and sophisticated ladies and gentlemen attired in Edwardian finery mill about, occasionally posing for photos at the behest of passing tourists. “Ooh, is it Downton Abbey that they're filming?” one excited passer-by inquires to a trio of costumed young women, who giggle in unison and reply in the negative. But the actor who's sprung from that series's stuffy parlours to become a fully-formed member of the global fame academy is close at hand for this Sunday morning shoot of period drama Summer in February. Dan Stevens is in the house.

“The Edwardian milieu is my speciality right now,” concedes Stevens, hidden inside the club's Greek-inspired exterior away from the eyes and ears of the fans straining to catch a glimpse. (Word has spread quickly that he's here; fan frenzy has become an everyday part of this actor's life.) “Although the plan was always to do this long before Downton.”

Based on true events, Summer in February is a calamitous love-triangle set in a bohemian artists' colony in Lamorna Cove in Cornwall, shortly before the outbreak of the First World War. Dominic Cooper stars as the colony's charismatic leader, British painter Alfred James “AJ” Munnings, while Stevens takes on his first major film role as Munnings's friend Gilbert Evans, the army officer overseeing the Cornwall estate where the artists live, and Emily Browning plays Florence Carter-Wood, the troubled young artist caught between them. It's based on the book of the same name written by Jonathan Smith, a retired English teacher who taught Stevens at Tonbridge School in Kent and recalls his precocious pupil marching up to him at the tender age of 13, demanding to go up against 18-year-olds for the role of Macbeth in an impending school production.

“That's absolutely true,” insists Smith, sitting in a red leather chair in the Athenaeum's imposing library. “When Dan auditioned, it was one of those tingly moments. Downton's made him famous but put that on one side: he's a seriously gifted actor. For me, it's just sheer good luck that he happened to be at the school I was teaching at.”

When I later stroll into the Athenaeum's sun-dappled gardens to find Stevens, he's practising fly-fishing moves for a scene to be shot the next day. He laughs when I recount his early mentor's recollections: “He remembers it very differently from me. I'm sure I offered to play a lesser role.” Stevens is dismissive of the notion that he was ever that bolshie, but praises Smith for moral support and pep talks during his adolescent years as a boarder at Tonbridge. “He persuaded me to go to Cambridge and read English. Even though he knew I wanted to be an actor, he knew that was a good call. And it was. All the way along, he's just quietly given me very wise advice.”

Although his education suggests otherwise, Stevens has always refuted notions of poshness. Born in Croydon, where he was quickly adopted by two teachers (he's never felt the urge to seek out his birth mother), he went to Tonbridge on a scholarship and his two career-thrusting roles have come as middle-class boys who end up moving in upper-crust circles: Nick Guest in the BBC's 2006 adaptation of Alan Hollinghurst's The Line of Beauty and, of course, Matthew Crawley in Downton Abbey. But it was at Cambridge that Stevens was first spotted, in a Footlights production of Macbeth opposite Rebecca Hall. Sir Peter Hall, the actress's father, went on to cast Stevens in several productions, including As You Like It in 2005, a role that landed him the Ian Charleson Award. Adhering to his tutor's advice, Stevens has also put his English literature degree to good use, founding the literary quarterly The Junket and sitting on the 2012 Man Booker Prize judging panel.

Stevens was well aware of Smith's 1996 novel, which had made the teacher something of a local celebrity in and around Tonbridge. “Everybody said, 'this should be a movie,'” recalls the actor. “I remember Jonathan jokingly saying, 'If it ever does become one, you'd make a great Gilbert'. I kept that in the back of my mind.” When a fellow Tonbridge pupil, Jeremy Cowdrey, later acquired the film rights, he tracked Stevens down and convinced the actor to attach himself to the project as both producer and star. This was a couple of years before Downton Abbey, and Stevens has loyally stuck with it through thick, thin and multiple writers (they eventually hired Smith to adapt his own novel).

As Gilbert Evans, Stevens is portraying a sensitive, thoughtful, quiet man opposite Cooper's loud and charismatic extrovert. “I guess it's one of those characters I play a fair deal, which is the outsider coming in and getting caught up in things,” he muses. There are question marks over what really happened, since Carter-Wood has essentially been erased from history. Thirty years after Lamorna, Munnings became a controversial head of the Royal Academy of Art (he's remembered for an incendiary departure speech in which he blasted Modernism and the likes of Picasso); he also wrote a three-volume autobiography in which he didn't mention his first wife once. As Gilbert and Matthew Crawley are cut from the same cloth, was Stevens never tempted to give himself the showier role of Munnings? The 30-year-old actor laughs, before admitting hesitantly, “Well, maybe. But I don't think I would have pulled if off nearly as well as Dominic. I'll get to those roles in the fullness of time. Gilbert's quite a good role for me to start out in films with.”

Summer in February's £5m budget was raised through private investors, and presumably among them were several Downton Abbey fans. It's clear that the show's success has made Stevens a bankable film commodity.

“It's sad to put it in those terms,” he shudders, “but I suppose that's right. Although Gilbert is number three on the cast list so I was just about eligible to play number three. Dominic seemed like the right kind of rogue to play Munnings. It's one of the nicest casts I've ever worked with. We've been having a blast.” Cooper will later tell me that the cast immersed themselves in the wild, hard-drinking bohemian spirit of their characters during the Cornwall leg of the shoot. Stevens is either too polite to share such tales of on-set debauchery, or didn't partake in these drink-fuelled bonding sessions.

When I spoke to him, he was pulling double-duty on Summer in February and the season-three shoot for Downton Abbey. He wasn't getting much sleep but didn't let slip that he had already made up his mind to leave the wildly popular series at the end of his three-season contract, still months off ruining Christmas for several millions fans when Matthew Crawley crashed his new car in the last scene of the show's festive special. Downton writer and creator Julian Fellowes later told The New York Times he had wanted to end the Christmas episode on a happy note and kill off Matthew in the first episode of season four, but that Stevens refused to hang about. “I didn't want his death to dominate the Christmas special,” Fellowes said, “so that's why we killed him at the very, very end.” Displaying less pique than Fellowes, Stevens explained after the episode's airing that he'd made the decision to go before season three even started filming. “We were always optioned for three years,” he explained. “It felt like a good time to take stock… I wanted a chance to do other things. It's a very monopolising job.”

There was no hint of Downton discord on Summer in February. Stevens expressed hope that the explosion in the show's popularity in the US would help the film's prospects, and praised Downton as “a fantastically valuable experience and a great ride. I've absolutely loved it.” He also gave a sporting chuckle when I raised some of the nuttier plotlines foisted upon Matthew, for instance his miraculous recovery from paralysis and impotence in season two. “I perform what is put in front of me!” he said. “There are some crazy storylines. We only get two scripts at a time so you never quite know where it's going. I'm not sure Julian knows quite until we start shooting and then by episode eight, he may have changed his mind about a few things.”

Stevens also excitedly told me that he'd recently been signed by powerful American agency WME. “When signing with someone becomes a headline, that's quite cool, although I was a bit like, 'Really? That's news?',” he laughed. “I'm very, very excited by those guys and they are excited by me. Things will start to heat up, which is exciting. Watch this space, man.” And indeed, since wrapping on Summer in February and leaving Downton, Stevens has played a heroin trafficker in the Hollywood thriller A Walk Among the Tombstones and journalist Ian Katz in the Wikileaks biopic The Fifth Estate; delivered an acclaimed stint on Broadway opposite Jessica Chastain in The Heiress; and become a father for the second time. His wife is Susie Hariet, a South African jazz singer-turned-teacher.

Stevens is now ensconced in New York with the brood, and his aspirations to break America have seen him hire a personal trainer and shed that Downton plumpness in favour of a new slimline look. The world is his oyster, although it remains to be seen how big a part Summer in February plays in his ceaseless rise. Back inside the Athenaeum Club, Stevens sits at a lavishly decorated wedding-banquet table and sulks while Cooper, as Munnings, makes an ungallant speech that sends his new bride storming off in tears. Getting up in between takes, Stevens wanders over to mingle with a few of Summer's investors, displaying the easy, smiling charm that looks set to take him to ever greater heights.

“It's an unusual one,” he muses. “It's not what they call in America 'a Thanksgiving weekend family comedy'. But I think it's a really beautiful, intriguing and heartbreaking story. I like those kind of soulful, tragic, romantic characters. But, yes, maybe a comedy next.”

'Summer in February' is released in the UK on 14 June

*This article appears in tomorrow's print edition of Radar magazine