Pedro Pascal (Oberyn) and Hafthor Julius Bjornsson (The Mountain) break down the epic shoot for THR: "There were retakes upon retakes to hone the scene to perfection."

[Warning: Spoilers ahead for Sunday's episode of Game of Thrones, "The Mountain and the Viper."]

Westeros' most epic trial by combat is over.

For book readers, this was among the most anticipated episodes of season four, with Oberyn (Pedro Pascal) and The Mountain (Hafthor Julius Bjornsson) fighting on behalf of Tyrion and Cersei, respectively.

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The fight was savage onscreen, and creating it was nearly as difficult for the actors involved. Even Bjornsson, known for competing in World's Strongest Man competitions, describes the shoot as "brutal."

"There were no holds barred, and there were retakes upon retakes to hone the scene to perfection," he tells The Hollywood Reporter.

Adds Pascal: "It felt as epic as it is in the story."

Hafthor, you are literally one of the world's strongest men. Did you have any worries about hurting Pedro while filming?

Hafthor Julius Bjornsson: We had to be careful during rehearsal, but during the fight itself, while filming, we had to go all in and that is where all the hard work paid off so no one got hurt, but we were able to create this amazing fight.

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What was it like being in the middle of this huge set piece?

Pedro Pascal: It felt as epic as it is in the story. We were in this really beautiful location in Dubrovnik, Croatia, overlooking the Adriatic Sea. The design team had created this incredible arena for a battle right on the water with these immaculately dressed extras. It was breathtaking to be a part of. The show upped its game as far as its own production design — which it's already incredible at.

What kind of fight training did you do to prepare for the big duel?

Pascal: They were kind of enough to send me to a Wushu trainer in Los Angeles before flying out to start shooting. It was certainly movement my body was completely unfamiliar with. They started me off there, and while I was on set, there were extensive fight rehearsals with fight choreographers and Hafthor. I was in Ireland and Croatia through most of the shoot of the fourth season, and so there was downtime for me to rehearse and train.

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Bjornsson: I started practicing in Iceland but then did several weeks' worth of training in Ireland and Croatia. Being trained by the swordmaster C.C. Smiff was fantastic but some of the hardest work I've done.

That sword looks gigantic. Was it heavy for you?

Bjornsson: Wielding the sword all day long is tiring, to begin with it's so heavy there are only a few who can use it. But during the fight itself was especially intense -- brutal. There were no holds barred, and there were retakes upon retakes to hone the scene to perfection. Naturally that is exhausting.

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Did you do all of your own stunts or were there times someone doubled for you?

Bjornsson: (Laughs.) How are you going to find a guy of my size and build to be my stunt double? No, I did this all myself.

What was the audition process like?

Pascal: When I self-taped my audition, to my enormous surprise, [showrunners] David [Benioff] and Dan [Weiss] got in touch with me via email. They started talking to me about the character, about what motivates this person emotionally, and the way that he lives his life and how linked it is to a specific event in his arc.

Bjornsson: I have to admit I was a bit nervous. I had no clue as to what I was going up against, but then it went really well. We read dialogue from the scenes, but then they wanted me to show them if I was fast enough for the fight scene. They were actually surprised at how fast I was, but I am naturally fast and on top of that I have practiced sports where you have to be very agile — and I think most realize I am strong.

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Hafthor, we got to see The Mountain fight The Hound in season one, but the fight was cut short. Who do you think would win in a real fight between them?

Bjornsson: The Mountain is older and has fought more, and I think he is a better fighter of the two. The Hound, even though he has a lot of heart, has weaknesses, like when he was saying that the worst part of being burnt in the face was the fact that it was by his brother. The Mountain is a sociopath, so he would stop and nothing and I believe he would win.

For more on the fight, read our Q&A with director Alex Graves.

Email: Aaron.Couch@THR.com

Twitter: @AaronCouch