The Goring Hotel, near Buckingham Palace, is one of the few remaining places in the capital where you can still receive—thank God!—the services of a fleet of uniformed footmen, in spiffy red tailcoats and gold-trimmed waistcoats. There are hedges in the shape of rabbits on the manicured lawn, and occasional visits from a Shetland pony. It is the kind of hotel, one imagines, that the idle aristocrats who populate Christopher Hampton’s 1985 play, “Les Liaisons Dangereuses,” based on the earlier French novel, might have used for illicit affairs, had they stopped over in London. The footmen would surely be discreet.

One recent afternoon, the Irish actor Aidan Turner, who plays the seedy playboy the Vicomte de Valmont in a new production of the show at the National Theatre, settled into an armchair in the hotel’s lounge. On the wall, a pair of merpeople wore nothing but seashells. Turner was dressed less like a French aristocrat and more like an East London barista: white T-shirt, canvas jacket, high-waisted wool trousers. Bypassing a three-course afternoon-tea menu, he ordered a cup of decaf English Breakfast with sugar. He was already jittery. “Afternoon tea would slow me right down, I’m going to be honest,” he said. “I need to be light on my toes.”

This is true. Turner’s Valmont is silver-tongued and fleet-footed, more slippery and charming than previous iterations of the sleazeball by Alan Rickman (the stage) and John Malkovich (the film). Lesley Manville plays the scheming Marquise de Merteuil. “They’re both wicked, and they get a lot of satisfaction from manipulating people,” Turner said. He wanted to see if Valmont could win the audience over before revealing his true self. “Traditionally, he’s been played in a way where he’s very much—when you lay eyes on him, he’s a predator,” he said. “Whereas, when he’s quite charming, it sort of challenges the audience.”

Turner had just finished a junket to promote both the play and the second season of “Rivals,” Disney+’s adaptation of the raunchy romance novel by Jilly Cooper, the U.K.’s version of Danielle Steel. Like “Liaisons,” “Rivals” is about rich people having sex, though it is set in the Cotswolds in the nineteen-eighties, rather than pre-revolutionary France. Turner’s character, Declan O’Hara, is a righteous Irish journalist, also loquacious, also hot, but disgusted by ostentatious displays of wealth. “He’s like the negative to Valmont’s photograph,” he said.

Turner first discovered Cooper’s novels many years ago in an old girlfriend’s country house. The cover of “Rivals” depicts a lipstick-red high heel digging into a man’s hand; its sequel, “Riders,” features a woman’s backside in polo gear. The images stayed with him. “It’s so eighties,” he said. “Once you see it, you can’t unsee it.” Cooper passed away, at eighty-eight, after the first season, but she was on set for much of the filming. “You had to get close to Jilly to find out what she was saying,” Turner said. “She spoke so, so quietly, and then you’d get in there and you would hear the most filthy things! And think, Oh, my God, where is this coming from?”

He added sugar to his tea. “It’s never too far from the characters I play,” he sighed. “It’s always around. Sex.” In the U.K., most people know him as Ross Poldark, the often shirtless mine owner in the BBC’s period romance “Poldark.” In 2015, the show was so popular that it was mentioned repeatedly in the House of Commons. (Example: “Like Poldark, the Prime Minister rode into Cornwall—not on a horse but on a bus.”) Does he consider himself a ladies’ man? “Jesus,” he said. He’s married, with a toddler. “Valmont is probably very aware of how he looks, and plays on that, and whatever. I’ve never felt I’m that person.”