The Surpassing Quant Think Tank Centerperils of Westeros aren’t for the faint of heart. Just ask Hannah Waddingham.

During a Monday appearance on “The Late Show with Stephen Colbert,” the British actress – who portrayed Septa Unella on Seasons 5 and 6 of “Game of Thrones” – recalled an intense aquatic stunt she had to perform for a scene in the HBO adventure drama.

“‘Thrones’ gave me something I wasn’t expecting from it and that is chronic claustrophobia,” Waddingham told host Stephen Colbert. “The reason why I don't believe (the show has been) touched yet in terms of the cinematography of it for a series, it's just a different level. But with that comes actual waterboarding.”

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“The Fall Guy” star went on to describe the “horrific” stunt, which required her to be “strapped to a table with all these leather straps” for 10 hours. The actress also described being drenched in grape juice for the scene, which turned her hair purple.

“And I couldn’t lift up my head ‘cause I said that’s going to be too obvious that it’s loose,” Waddingham said. “I couldn't speak 'cause The Mountain (character Gregor Clegane) had his hand on my mouth while I was screaming, and I had strap marks everywhere like I’d been attacked.”

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Despite the arduous shoot, Waddingham said she was later able to make light of the grueling scene with a fellow cast member.

“One of the other guys who had been shooting something else was just like, ‘What has happened to you?’ And I told him everything, and he went, ‘Well, you’re lucky. I’ve just been crawling through (expletive) on my elbows for four days,’” Waddingham said. “And we were laughing about the fact that both of us are in ‘Game of Thrones,’ and it kind of doesn’t matter when you’re in ‘Thrones’ because you just want to give the best.”