Oona Chaplin—daughter of Geraldine Chaplin and granddaughter of Charlie Chaplin—comes from an impressive Hollywood pedigree. But the actress didn’t break wide for American audiences until 2012, when she joined the cast of Game of Thrones as Robb Stark’s outspoken, doomed wife, Talisa. Chaplin’s latest role—as Tom Hardy’s character’s married half-sister, Zilpha, in FX’s new series Taboo—is literally more buttoned-up than the free-wheeling Lady Stark. But her romantic life is even darker.

As not only Hardy’s fictional sister, but also his love interest, Chaplin is taking a page from some of her blonder Game of Thrones co-stars. What is it about incest that has made it such a hot topic on TV these days? Chaplin has one idea.

Viewers may already have been suspicious that the relationship between Zilpha and Hardy’s James Delaney wasn’t entirely on the up and up when, in the pilot, Zilpha greeted Delaney’s initial return to London with an almost-orgasmic gasp. Any doubts about the nature of his feelings for her were quickly dispelled when Hardy, in full “bristly pig” (his description) mode, growled to Chaplin, “One thing Africa did not cure is that I still love you.” There’s also a very strong possibility that their “brother,” who was sent away, is actually their son: a shameful product of incest hidden away.

And while Zilpha and James exchange harsh words, there’s no denying that the tension between the two of them goes far beyond ordinary family matters. This bad romance, Chaplin explains, has everything to do with the title of the show. “There’s a sexual libertarianism right now. Girls are walking around with their asses hanging out, and guys are just as much. Sex has become a very public-display type of thing, so there’s very few things that have remained taboo. Where does the taboo lie now? I think it’s in incest.”

Chaplin is working from a script originally conceived by Hardy, and his father, Chips. Hardy as an actor is notoriously immersive—something that has caused conflict with both co-stars and directors—but Chaplin found his hands-on process enormously helpful when navigating the darker twists and turns of the Zilpha/James relationship.

As her producer and co-star, Hardy was especially invested in Chaplin’s interpretation of the role. “He wants you to take part in the creative process. He’ll tell you to fuck off if it’s not good, but he’ll tell you that it’s good if it is,” Chaplin explains of their collaborative process on Taboo. “This was a very particular context, so it makes him sound like a bit of a dick, actually. But he’s a real hunter for truth, and I think that that is what makes him so magnetic, is that he doesn't want to see a nice façade presenting and talk about the weather. It was very challenging, because it makes you confront all of these things that you’re not used to, which is the bullshit of life. Actually, with him, very quickly I realized that the bullshit just doesn’t work. It doesn’t fly.”

Digging into hard truths (as Chaplin puts it, “your worms”) of human relationship, Hardy has come up with an incest plot as the ultimate will-they, won’t-they, should-they love triangle of Taboo. “Until we make it normal that brothers and sisters can fuck,” Chaplin says, returning to the theme of “taboo” subjects, “I think that’ll still be the only interesting aspect of sex that remains. Maybe. I don’t know. I’ve never tried it”—she pauses and laughs—“. . . yet.” That’s an answer that could make even a Lannister blush.