​Rose Leslie on being Idris Elba's co-star in 'Luther' and life after Game of Thrones

Rose Leslie, 27, has landed in some major television shows since leaving drama college in 2008. She was the self-improving housemaid Gwen Dawson in the first series of Downton Abbey, which got her noticed by the producers of Game of Thrones, who cast her as the wildling Ygritte. After three seasons in HBO's fantasy phenomenon, she is now playing Idris Elba's new police sidekick in the returning BBC1 police drama Luther. It's like a jackpot with each pull of the slot machine.

“I'm a big fan; I think it's truly superb,” she says of her new gig when we meet in a Soho members club. “And because I'm such an avid fan I was unaware that Luther was going to come back for another series. I thought that chapter was closed with Luther throwing the coat into the river.”

She is referring to the last time we saw the detective, over two years ago now, chucking his outerwear into the Thames to signify the end of his career as a copper, before slouching off into the sunset beside many people's favourite psychopath, Alice Morgan (Ruth Wilson).

She had no idea that Game of Thrones was a global phenomenon when she auditioned for the part of bow-and-arrow wielding Ygritte in 2012. “Thank heavens I was that naive because I think if I was aware of the fandom behind it – and not just the first season but also the books – I think it would have thrown me,” she says, going on to describe the audition with an attractively earthy chuckle.

“Oh my gosh, I walked into the room and I was surrounded by gingers. Every single ginger actress who was in London was in that room, and I just wanted to celebrate with them all.”

Rose Eleanor Arbuthnot-Leslie is her given name, her father – a financial head-hunter – being the Aberdeenshire chieftain of Clan Leslie, and she was raised in a castle surrounded by “puddles, bushes and trees... digging worms at the bottom of the garden... I feel incredibly privileged where I was brought up”.

She is the third of five siblings, and she says she spoke with a Scottish accent when younger (“I was always 'wee Rosie'”) but lost it when her English-born mother, Candida, wearied of the Aberdeenshire weather and took the family off to Paris for three years.

Millfield in Somerset inculcated a love of acting, and cemented her public school English tones. But it was a Scottish accent that helped to win her first TV role, while still in her final year at Lamda, playing a young woman from the Outer Hebrides in Annie Griffin's quirky comic study of Edinburgh types, New Town. She won a Scottish Bafta for her performance.

She spoke in a Yorkshire accent as Downton's Gwen Dawson, the under housemaid who took a correspondence course and went off to work as a secretary at the end of the first series, returning in the most recent as Mrs Harding. “I was clapped back on stage when I went back,” she says.

Her Game of Thrones character, Ygritte, was killed off at the Battle of the Wall, but one thing that didn't die was the speculation of a relationship between the actress and Kit Harington, who may or may not still be playing Snow after his character's “death” in the most recent series. “We're having a relationship, are we?” she says. “I'm a naturally guarded person anyway, and that's a Pandora's Box if you were to divulge. If it's something that should be private and is very dear to your heart, you make an effort to keep it that way.”

She is stopped a lot on the street (“It's the ginger hair that gives me away, but people are so passionate about Game of Thrones that it's never a nasty incident”), but she stays clear of social media. “I'm completely unaware what the trolls are saying about me,” she says. “I prefer being private.”

And what she is up to is mostly taking place in America, where she filmed a well-received, low-budget horror film, Honeymoon, in North Carolina, and played Ray Liotta's daughter in the New Orleans-set Sticky Notes. “I watched Goodfellas on the flight over, and maybe I shouldn't have because meeting him [Liotta] a day later I freaked myself with how brilliant he was,” she says “But he was very welcoming, giving me advice and ideas for the script.”

Leslie has also finished filming a sci-fi movie, Morgan (the directorial debut of Ridley Scott's son, Luke), playing a scientist who cares for the world's first genetically modified human. But now she is resting at home in north London, reading scripts and waiting to hear back about new projects. “I'm making sure that the next choice is right,” she says. “To make a concerted effort to look for a character that I'm not artistically repeating myself.”

She is acutely aware of the inequalities of her profession. “My guy friends from Lamda are doing better than the girls in the sense there are more male parts out there,” she says. “I think there's a turning of the tide though, which is exciting for me and my girlfriends. The Amazons and the Netflixes of this world are changing the medium... female parts with backbone and not a sexualised role, but more headstrong.” Which, in a nutshell, is a pretty good description of all her roles so far.

'Luther' returns to BBC1 on Tuesday at 9pm