One of the stars of 'Game of Thrones' has spoken out about a horrific scene she had to shoot which involved being waterboarded for real.

Those who watched 'Game of Thrones' may recall how one of the storylines involved the cult-like Faith of the Seven, who served under the High Sparrow (Jonathan Pryce).

Unella (Hannah Waddingham) served as one of the Septa of the religion. She tortures Cersei Lannister (Lena Headey), famously shouting "shame" while ringing a bell during Cersei’s walk of atonement through the streets.

But once Cersei regains power she gets her own back. Unella is imprisoned and the last the audience sees of her, the woman is tortured by Cersei and then left in the hands of the Mountain.

Waddingham explains: "She was meant to be raped by The Mountain, and I think they’d had so many complaints about the rape of Sansa that they chose not to go with it."

The actress only learned that the scene was changing on her flight to Belfast to film the 'Game of Thrones' episode.

She says: "I got sent these new sides that said that I would need a wetsuit top. And I thought they'd sent me the wrong bits.

"Sure enough, when I got there, I was then put in a wetsuit top and I was like, 'Because?' And they went, 'Oh, it's waterboarding instead.'"

When Waddingham questioned the 'Game of Thrones' production team if they were really waterboarding her for the scene, they said: "No, no, no. We are."

Waddingham describes the experience in detail: "Here I was strapped to a wooden table with proper big straps for ten hours. And definitely, other than childbirth, it was the worst day of my life. Because Lena was uncomfortable pouring liquid in my face for that long, and I was beside myself.

"But in those moments you have to think, do you serve the piece and get on with it or do you chicken out and go, 'No, this isn’t what I signed up for, blah, blah, blah?'

"And then, the funny thing was, after we'd finished shooting it for the whole day, and people like Miguel Sapochnik, the director by the way, walking past with a cup of tea and a sandwich on-the-go and going, 'Hi hunny, you alright?' And I was like, 'Not really.' 'The crew have just been saying we are actually really waterboarding you here.' And I was like, 'Yup, you don't need to tell me that!'"

She adds: "I could barely speak because I had been screaming through The Mountain's hand, which is quite frightening as a singer to completely lose your voice, so I had no voice at all to barely whisper, bruises already coming up like I had been attacked and I was like, 'I've basically just been waterboarded for ten hours.'"

The actress added that shooting the 'Game of Thrones' scene gave her aquaphobia and she subsequently couldn't watch a programme that involved an actor's face being dipped in water.

She continues: "I actually went and had a bit of a chat to somebody about it, because it's quite full-on being waterboarded for ten hours, and then only one minute and 30 seconds can be used on camera."

You can read Hannah Waddingham's full interview with Collider here.