Hannah Waddingham claims filming some of the more harrowing scenes for Game of Thrones left her with claustrophobia.
The Ted Lasso actress, 49, portrayed Septa Unella in the HBO hit fantasy series.
In season six, Unella is tortured by a vengeful Cersei Lannister (Lena Headey), who pours a jug of wine onto her.
Recalling her experience while appearing on The Late Show with Stephen Colbert, she began: "Thrones gave me something I wasn't expecting from it and that is chronic claustrophobia.
"I have talked about it since with David Benioff and Dan Weiss, the two exec producers on it, I was like, 'Good job it's for them because it was horrific.'"
"[I had] 10 hours of being actually waterboarded, like actually waterboarded," she continued matter-of-factly, while Colbert was visibly shocked.
Waddingham then praised the show adapted from George RR Martin's series of novels, though she admitted filming proved to be challenging at times.
"The reason why I don't believe [Game of Thrones] is touched yet in terms of the cinematography of it for a series, it's just a different level," she said, adding: "But with that comes actual waterboarding."
Waddingham recalled being "strapped to a table" and then later laughing it off with a co-star as they compared similar on-set horror stories.
"So I'm strapped to a table with all these leather straps and I couldn't lift up my head cause I said, 'That's gonna be too obvious that it's loose, I was like, right? I quite like it to be loose,'" she said.
"So I'm on my way back and I'm in this fancy lift and my hair is already bleached to death, I had grape juice all in my hair so it went purple.
"And I couldn't speak 'cause The Mountain had his hand on my mouth when I was screaming, and I had strap marks everywhere like I'd been attacked.
"And the lift doors open, one of the other guys who'd 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 s*** on my elbows for four days.'
"We were laughing about the fact that both of us are in Game of Thrones. It kind of doesn't matter when you're on Thrones cause you just want to give the best [performance]."
