SPOILER ALERT: Do not read on unless you’ve seen the sixth episode of “Outlander,” titled “The Garrison Commander.”

The sixth episode of “Outlander” proved to be the most intense and harrowing to date — remarkable, considering that much of the hour focused on two characters simply sitting in a room and talking. The cat and mouse game between Captain Black Jack Randall (Tobias Menzies) and Claire Beauchamp Randall (Caitriona Balfe) took a number of unexpected turns, something that co-star Sam Heughan admitted he “loved watching” just from an audience standpoint, telling Variety during a conversation during July’s San Diego Comic-Con that “I was on tenterhooks; it just turns on its head all the time.”

Variety was on set for the filming of a portion of that intense scene — beginning with Claire’s meeting of Lord Thomas — a day that would cover 11 pages of the script. Balfe revealed that they’d been shooting the back-and-forth for three days prior, and would have another four to come. “It’s a very hopeful moment [when she meets Lord Thomas] and then Black Jack Randall comes in to thwart it all. But it’s great because it’s a real tete a tete. They really just go at each other and she can’t help it,” she laughed. “That’s the great thing about Claire, she just can’t help fighting back. Sometimes that’s not always the smartest thing but it makes it very interesting. And also a lot of the guys who are playing the redcoats, they are kind of new to the cast. And so it’s a whole different dynamic to what I’m usually used to. I’m usually with all the hairy highlanders — that’s what we call them — so it’s quite cool because it’s [now] well-spoken British people. And it’s a great thing because she thinks she’s on her way out and then everything’s dashed.”

When we caught up a few months later at Comic-Con, Balfe admitted that filming the episode “felt like we were doing a play because it’s such a long scene.” While initially, they had planned for more movement and blocking, the actors and director ultimately preferred the choice to keep the confrontation contained. “It’s just two people who sit and talk and it’s great that we did it like that. For me, it was just such a powerful scene; first of all, Claire sees that connection to [her husband] Frank there, a physical thing that she has held and touched … she felt like she had to be able to get through to this person,” she explained.

Sadly, just when Claire thinks she’s found a spark of humanity in her husband’s ancestor, he delivers a literal punch to the gut, stunning both Claire and the audience.

“It’s a great twist, I think,” Menzies said at Comic-Con. “Watching it, I’d completely forgotten that I beam at her just at that moment — there’s that big broad smile and I hadn’t remembered doing that, and it’s a great moment. You go, ‘oh wow, she’s gotten through to him,’ and to understand that it’s all deception is heartbreaking.”

Though Heughan’s character, Jamie Fraser, only appeared in two brief scenes in the episode, both were pivotal. The first, his brutal flogging at the hands of Black Jack, was in many ways the catalyst that drives everything that follows — including the decision that he and Claire should marry to protect her from the barbarous captain.

Of filming the scene when Black Jack takes the whip to Jamie, Heughan recalled, “It was hard work to do. It was freezing cold there — I think it was February, I had this whole blood rig on, I was manacled and hanging from the stage… I really wanted to see Jamie lose almost everything, but not. Tobias, he goes for it, it was wonderful. I love the bit where he slips in the mud, and when the back starts flaying off. It had to be visceral, because there’s a guard that faints, so it has to be epic and it has to be the reason he has these scars on his back that becomes this [driving force]. I’m so pleased it turned out so well.”

Menzies agreed, “It was a mixture of both intense and very technical — there’s a lot of time where you’re flogging with a wooden thing in your hand and they just CGI in something later, and times when Sam is subbed out and I’m just hitting this plastic. Mainly, it was a physical proposition, it had to be a very visceral, guttural [display] of malevolence, really. It’s important because it’s a driver throughout the story, it’s something that Jack refers back to, something that Jamie is scarred for the rest of his life from. Both of their lives are not the same after, so it felt like an important thing to get, the weirdness and mania of whatever that event was. But it’s definitely a day where you have to commit massively to what you’re doing.”

While many actors might have struggled to hold on to the character in the midst of such specific technical work, Menzies admitted, “I find those bits of acting almost easier than the little scenes where you chat about tea or something, because you can just submerse yourself and commit to it. They’re certainly the days that I got into acting for, to pit yourself against making that believable.”

“Outlander” airs Saturdays at 9 p.m. on Starz.