The 'Succession' writers got the idea for Roman and Gerri's relationship because Kieran Culkin and J. Smith-Cameron would flirt 'in the most obnoxious way' on set

• J. Smith-Cameron and Kieran Culkin play Gerri Kellman and Roman Roy, respectively, on "Succession."
• Smith-Cameron told THR that the costar's off-screen dynamic inspired their character's relationship.
• "I would flirt with J. in the most obnoxious way, just to fuck around," Culkin said.

HBO's Emmy-winning dramedy "Succession" is filled with twists, perhaps the most shocking of which is the bizarre relationship between Roman Roy (Kieran Culkin) and Gerri Kellman (J. Smith-Cameron) that emerges in season two.

Roman, an heir to fictional media conglomerate Waystar Royco, and Gerri, the company's general counsel, are an unlikely match for a variety of reasons, from their age gap to their positions within the business. During season two, they engage in closed-door conversations, during which Roman masturbates to Gerri's scathing insults.

"Succession" writers gained inspiration for the twisted Roman-Gerri storyline after watching Culkin, 39, and Smith-Cameron, 64, interact on set.

"I would flirt with J. in the most obnoxious way, just to fuck around," Culkin told The Hollywood Reporter on Wednesday.

While filming the season-one finale "Nobody Is Ever Missing," centered around Shiv Roy (Sarah Snook) and Tom Wambsgans (Matthew Macfadyen)'s wedding, Smith-Cameron said she and Culkin both turned around to check out each other's butts.

The off-the-cuff moment captured surrounding writers' attention.

"I remember the writers all laughing," Smith-Cameron told The Hollywood Reporter, adding, "But it never occurred to me that it would inform anything they wrote."

Long before Culkin and Smith-Cameron joined the "Succession" cast, they were friends and collaborators.

They both had roles in "Margaret" and "The Starry Messenger," and Culkin appeared in "This Is Our Youth," a play written by Smith-Cameron's husband Kenneth Lonergan.

Culkin had long dreamt of seeing a sexual crossover between Gerri and Roman in the script but didn't expect the subplot to actually make it on-screen.

"I remember saying to my wife in season one, 'I really hope something happens sort of sexually or romantically between those two, but I don't know that it ever will,'" the actor said.

HBO seemingly confirmed that Gerri and Roman's complex alliance will continue in the show's third season, premiering October 17, by releasing a promotional poster of the duo together. Predictably, the image threw "Succession" fans into a frenzy online.

Like the show's viewers, Smith-Cameron is captivated by Roman and Gerri's unexpected trajectory.

"I have to say in my long career, I don't know that I've been in such an unusual story arc. It's so interesting to me, and complicated and kind of shocking - and it's sort of sad and fascinating in terms of #MeToo, and in terms of women of a certain age, flexing their power," she told Variety.

"It's got many facets that make it really topical."