Hannah played Septa Unella in the fifth season of the fantasy drama, appearing in one scene in which her character was tortured.
Speaking on Collider Ladies Night, Hannah claimed that as part of the filming process she was “waterboarded for 10 hours”, suffering both physically and emotionally in the aftermath of the scene.
She explained that initially her character was supposed to have been raped, but this was changed “quite at the last minute” to a torture scene instead.
“I think they’d had so many complaints about the rape of Sansa [Stark, played by Sophie Turner] that they chose not to go with it,” she said.
Hannah then arrived on set in Belfast and was swiftly given a wetsuit.
“They were like, ‘Oh, it’s going to be waterboarding instead’,” she said. “And I was like, ‘But we’re not actually doing waterboarding.’ And they were like, ‘No, no, no, we are’.
“There, I was strapped to a wooden table with proper big straps for 10 hours. And definitely, other than childbirth, it was the worst day of my life.
“[Co-star Lena Headey] was uncomfortable pouring liquid in my face for that long, and I was beside myself. But in those moments, you go, ‘Do you serve the piece and get on with it?’ Or do you chicken out and go, ‘This isn’t what I signed up for’?”
When the 10-hour day of filming was over, Hannah recalled telling director Miguel Sapochnik “not really” when he asked if she was OK, to which he responded: “The crew have just been saying we are actually really waterboarding you here.”
“I was like, ‘Yup, you don’t need to tell me that!’” Hannah added.
The actor said that when she returned to her hotel that night, her voice was hoarse from screaming and she was bruised, adding that the experience gave her “claustrophobia around water”.
“It’s quite full on being waterboarded for 10 hours,” she concluded. “And then, for only one minute and 37 seconds to be used on camera.”
HuffPost UK has contacted HBO for comment and is awaiting a response.
The British star can also be seen opposite Jason Sudeikis in the US comedy Ted Lasso.
Watch Hannah’s full interview on Collider Ladies Night here.