Every episode of Scenes From a Marriage begins with a behind-the-scenes glimpse of production, reminder of how the HBO show is a piece of filmed entertainment, being created by actors and a director and crew. In similar ways, the extremely intimate relationship drama about the disintegrating marriage of Jonathan (Oscar Isaac) and Mira (Jessica Chastain) blended the line between real and fiction more than once, especially when it came to Episode 4, when the brewing tensions between the two characters explodes into not just an emotional but also a physical confrontation.

When Collider's Christina Radish talked to Jessica Chastain and Oscar Isaac about the sequence, they used a variety of words to describe what occurs, including "messy," "painful," and "difficult." As Isaac said:

We talked about how it needed to feel not premeditated at all. Clearly, they’re not thinking about it. It just spirals out of control before they can even realize what’s happening. There’s an element of that. That’s why it was also important that alcohol played some role in what happened there. We just talked about it staying messy.

Added Chastain:

With everything on this, except for the heavy intimacy scenes, we really wanted it to be as real as possible. And so, for that scene, it was real. It sucked. We hurt each other. That’s a difficult thing, when you really care about someone, to be in a scene like that. It’s painful.

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When it came to execution, director Hagai Levi told Collider that unlike other scenes, they didn't rehearse the ultimate confrontation of the episode a lot, and that they'd brought in a stunt coordinator to oversee the action. But even with a stunt coordinator, he said, the nature of the production ensured that the scene would feel very raw in its execution:

We shot it chronologically, the whole series. We shot it the way it is. So, we got to this scene after the whole Episode 4, which was a very hard episode. When we get to that point, they just did it. They just did what they felt, at the same moment. It was totally real. Of course, they knew how to not physically hurt each other and knew not to kill each other. It’s hard to explain what kind of relationship they had during this shooting. It’s like they were a couple, coupled with the love and hate for each other for real. They couldn’t hide anything from each other because they knew each other so well.

Chastain said that she knew, going in, that this would be the most challenging episode of the series for her. "It was the one, when I was reading it, where I couldn’t really control the emotions as I was reading. I think it was the most difficult thing I’ve ever done, emotionally, just in terms of a scene."

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Image via HBO

To prepare, she said, "what I did before each episode was that I would decorate my dressing room with visuals that inspire me. I put up some images of where Mira might be internally, in her life, at that point. I just had to just go there. It was really super dark, sad to say. I think a lot of actors could say, 'Oh, yeah, I did it, and then I went home and it was fine.' That wasn’t me. I just knew what I was heading into. Even talking about it is difficult. It was really dark. "

In a moment of levity, Isaac then joked that "it was fine for me. I had a blast, smacking my dear friend across the face." But then, he immediately added, "No. It was heavy. It was difficult. It was a challenging episode to do, for a number of reasons."

One episode remains in Scenes From a Marriage, meaning we have one week to wait before finding out if happily ever after is even possible for these characters, alone or together. The finale airs Sunday at 9 p.m. ET on HBO.

KEEP READING: Oscar Isaac and Jessica Chastain on ‘Scenes from a Marriage,’ Being Each Other’s Acting Coaches, and Breaking the 4th Wall