King & Conqueror star recalls moment after training when he thought he was "about to die"

After training, the duo would "have to go and do what they do with these cold dips", explained Coster-Waldau, which Norton, a veteran plunger himself, described as "fun".

"I learned that," added his co-star. "I'd never done it before. The first time, you feel that you're about to die, and then you suddenly realise, oh, this is really nice."

Norton went on to say that it made their training "really, really enjoyable, because we would always couple it with this custom, which is to get to soak in cold water and then basically poach in warm water".

"And so Nikolaj and I would always go down, do our stunts, and then head out... we had to be on it. We're not that young anymore."

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Coster-Waldau, who is 15 years older than Norton, then recalled a scene in which the pair fight naked in bathtub.

"He's so much younger than me, he's so much stronger, so it's very unfair," he joked.

"That's not true," said Norton, before referencing Game of Thrones, Coster-Waldau's landmark role. "This man knows his way around a medieval set, as we know. He can swing a sword... that sounds euphemistic. [Laughs]."

"King & Conqueror is the story of a clash that defined the future of a country – and a continent – for a thousand years," reads the official synopsis, "the roots of which stretch back decades and extend out through a pair of interconnected family dynasties, struggling for power across two countries and a raging sea.

"Harold of Wessex and William of Normandy were two men destined to meet at the Battle of Hastings in 1066; two allies with no design on the English throne, who found themselves forced by circumstance and personal obsession into a war for possession of its crown."

In an interview with the BBC, Norton said that he was "constantly surprised" when reading the scripts "and saying, 'Is this real? This seems completely mad.'

"And invariably, he [lead writer Michael Robert Johnson] would be like, 'No, that is exactly what happened.' It was extraordinary."

Speaking to The Guardian, Norton remarked that he "wasn’t aware of the relationship Harold and William had before the battle, that they were friends and allies for many years, before they realised that, because of the way Europe was being carved up, they would both inevitably end up on a battlefield".

"And one of them would have to die," he added.

Alongside Norton and Coster-Waldau, the rest of the cast includes Emily Beecham (The Pursuit of Love, 1899), Clémence Poésy (Harry Potter, The Walking Dead: Daryl Dixon), Eddie Marsan (Ray Donovan, The Thief, His Wife and the Canoe), Juliet Stevenson (Bend It Like Beckham, Wolf), Clare Holman (Lewis, Sherwood), Luther Ford (The Crown, Black Doves), Bo Bragason (Renegade Nell, The Jetty), and Indy Lewis (Industry, House of David).

King & Conqueror premiered on BBC iPlayer and BBC One on Sunday 24th August.

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