The Diplomat creator and showrunner Debora Cahn approached the Netflix series’ key season three episode, “Amagansett,” with all the trepidation of someone hosting a diplomatic summit. “There was a lot of flop sweat,” Cahn says of her experience writing the pivotal hour.

Taking place at the private home of Allison Janney‘s President Grace Penn, it was a point the series had been building to for three seasons: the dissolution of long-term relationships both for its central couple (Ambassador Kate Wyler, played by Keri Russell, and Rufus Sewell‘s Vice President Hal Wyler) and the U.S.-U.K. diplomatic arrangement central to the narrative.

Amid this double breakup, another strained marriage emerges between Grace and the frustrated first gentleman, Todd, played by season three addition Bradley Whitford. Cahn says she was able to unlock her latest character by writing a scene in which he unknowingly bleeds on a platter of oysters — and serves them to the other three, creating an awkward situation. Cahn says she was so pleased with the farcical, play-like scene, it gave her the confidence to explore more dynamics among the quartet in season four, for which Janney and Whitford were upped to series regulars. “I remember groaning with pleasure when I read that scene,” Whitford says, with Sewell adding, “The dialogue was so on point, the story was so clear, it was so subtle. What was funny about it was also incredibly real.”

Embarrassed by his unsanitary faux pas, Todd is left by himself on the couch while the grown-ups decamp to talk political strategy.

“It’s an interesting character, a spouse who’s not a player in the room and what that feels like,” says Russell, who describes Whitford as a completely different instrument in their orchestra. “I love what he’s doing with the character. It’s so weird and specific. You still read that he’s very smart and had a big life. Now he’s just stuck in this purgatory of following [his wife] and all that that entails.”

Cahn wanted the episode to illustrate the differences in communication and power dynamics within marriages and professional partnerships. “We’re looking at the professional roles and how they graft onto the roles in the marriage,” she says. “What does it mean to be the host and Todd being put in a position where he feels like he has to wander into that role in a way that he’s not great at? The presentation of the oysters is him trying to gamely soldier forward, and he bungles the whole thing — or at least is treated like he’s bungled the whole thing by his wife.”

Speaking as The Diplomat shoots its fourth season, one that’s shooting in Italy, the U.K. and New York, Cahn and the cast promise more interactions between the Penns and Wylers. Filming in Florence during the last week of May, the actors were again working as a quartet.

“It’s all these dynamics now at a different place in their lives and in our lives,” Whitford says of how the scene he had filmed that day compared to the oyster incident.

Janney and Whitford famously worked together on The West Wing, where Cahn served as a writer for the NBC political drama’s final four seasons, an experience Janney says anchored their onscreen reunion. The roles they play in The Diplomat actually came down to a matter of timing.

“[Janney] was unavailable for us in the very beginning,” says casting director Julie Schubert, who had her eyes on both performers. “She was on many, many, many things. We’d also been talking about [Whitford] from day one and finding a way to bring them into this world in an exciting and unexpected way.”

As season three goes on, Todd’s feelings of being left out lead to his suspecting that his wife might be having an affair with the vice president.

“On the one hand it’s this completely ridiculous jealousy that you know is kind of objectively silly,” Whitford says. “But there’s something really human about it and his sad willingness or need to express it. I find him impossible and heartbreaking at the same time. I have a lot of affection for Todd and the difficult position that he’s in.”

Janney suggests that Todd’s jealousy continues in the upcoming season. “It’s in his head for a reason,” she says. “It’s not necessarily that they’re having an affair physically, but definitely emotionally they are finding themselves aligned with each other. They are jiving in a way that politically is making them very powerful. I think that’s very intoxicating. And I can see why Todd and Kate would be a little threatened by it, but it doesn’t necessarily mean they’re bedfellows.”

Russell agrees about the connection. “If Kate notices anything, it’s the intimacy of the work, which is what she is so attracted to and wants to be a part of,” she adds. “And I think she is able to acknowledge that there is an intimate working relationship that she is not a part of.”

Sewell is a bit more vague but urges viewers to stay tuned. “I wouldn’t like to open or close any doors imaginatively for the audience,” he says. “Things are not necessarily as they seem.”

Romantic tension, real or imagined, is only half of The Diplomat. From a foreign policy standpoint, “Amagansett” sees Grace assessing whether to tell the U.K. that she was involved in the fictional British carrier ship attack that kicked off the series. But when U.K. Prime Minister Nicol Trowbridge (Rory Kinnear) goes rogue and announces to the press that dead former President Rayburn (Michael McKean) was behind the attack, protests break out in the U.K. as the trust between the two countries is thrown into jeopardy. Cahn says she plans to continue to explore the consequences of the attack in season four, maintaining it as a thread throughout the series.

Janney says that the carrier fallout comes up again in season four, as Grace again makes questionable decisions. “It’s pretty wild,” says Janney. “Grace is not afraid to make bold decisions and to suffer the consequences, whatever they are, and she’ll stand behind what decision she makes. Not everyone agrees with her.”

The carrier, in some ways, is The Diplomat‘s Archduke Franz Ferdinand.

“There was always a very long plan for what the carrier meant to the relationship between these two countries,” Cahn explains. “The consequences reverberate through the third season and will continue through the fourth. It’s satisfying for me to be able to make one geopolitical error resonate over a number of seasons. Look at the relationships between countries. We are still dealing with things that happened decades, sometimes centuries ago. Certainly with the U.K., the origin story [of the U.S.] will always be hanging over the way that we interact now.”

Four seasons is a relatively long run in the streaming era, but Cahn insists she’s not yet nearing the end of this tale. “I grew up in broadcast television, where one season was 22, sometimes 25, episodes, so it feels like we’re just getting started,” she says. “It doesn’t feel to me like we’re in season four. It feels like we’re in the juicy middle.”

This story appeared in the June 16 issue of The Hollywood Reporter magazine. Click here to subscribe.