A bright spring morning in London, and Aidan Gillen is standing outside a Soho club being photographed for Esquire. Some passersby stop to see what the fuss is about and take snaps of their own — Game of Thrones cast members command a high approval rating on social media — but Gillen seems unfazed. No wonder, perhaps: the location was his idea.

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"I knew I'd feel comfortable hanging around a few doorways in Soho in something that looks cool," the 49-year-old Irishman tells me later. "I've loved it here ever since I came to London in the late Eighties. When you're 19 and working in theatre, you finish at 11 o'clock and don't want to go home. So, I'd hang around here with all the gangsters and musicians and transvestites."

Since those days, Gillen has clocked up 18 years starring in landmark television, from 1999's groundbreaking drama Queer as Folk to TV-as-art touchstone The Wire to the aforementioned, really rather popular dragons and death fest. This summer, Gillen returns as conniving social climber Petyr Baelish, aka Littlefinger, one of the few characters to appear in all seven series of Game of Thrones and one of the fewer still who are genuinely complex.

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"If the audience isn't sure whether they like you, you're probably doing the job right," he says. "The amount of treachery I've been involved with now, they've got plenty of reasons to hate Littlefinger. But my job is to keep them on-side."

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Game of Thrones has made Gillen's face internationally recognisable, especially when he's in the company of his co-stars. "It's one thing for a Game of Thrones fan to see one cast member, but try having a game of pool in Belfast with Gwendoline Christie and Kit Harington…" he says. "They go crazy!"

There is little point pressing seasoned Game of Thrones cast members for spoilers — HBO keeps an assassin's arrow trained on them at all times — but Gillen does let a small hint slip when I mention audiences missing the scenes with his early sparring partner Varys (played by Conleth Hill). "Well, you might see that again, you never know…" he says with a trace of Littlefinger's sly smile on his lips.

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It's no stretch to suggest that the phenomenal success of Game of Thrones has made the film we're notionally here to talk about possible: Guy Ritchie's King Arthur: Legend of the Sword is a medieval fantasy epic with plenty of blood and thunder. "I see it as a gang caper, and my character is one of the gang," says Gillen of Goosefat Bill, the hitman who accompanies Arthur on his adventures. "Obviously, I'm supremely talented with a bow and arrow, which helped. And you also see a snake roar at one point, which I thought was a real innovation."

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He seems to be taking all this high-profile success in his stride. "My first director said, 'Don't be in a hurry to be getting to the top of the ladder, because the best bit is the journey.' Which I think is important whatever you do." Was that the best piece of encouragement he's had? "That and the people who said, 'Don't go off to London to try and be an actor, you're not going to make it.' That was encouraging, in a different way."

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This article appears in the May 2017 issue of Esquire

Interview by Sam Parker

Photographs by David Titlow

Fashion by Catherine Hayward

King Arthur: Legend of the Sword is out in cinemas on 19 May

Game of Thrones season seven begins on 17 July on Sky Atlantic