Usually as an actor, when you’re offered a role it’s pure joy,” says Jonah Hauer-King. When he accepted the lead in the new series The Tattooist of Auschwitz, playing the Slovakian Jewish prisoner Lale Sokolov, however, he was “terrified”.

“While I was incredibly honoured, I was aware of the responsibility and how important it was to do the story justice. I felt pretty daunted.”

The rising star, 28, is best known as charming Prince Eric in last year’s The Little Mermaid but this might change after this six-part series — about the true story of the Jewish prisoner who tattooed ID numbers on to fellow prisoners and fell in love with one, Gita, who became his wife — airs next month.

In the hotel where the series is today being promoted, Hauer-King wears a preppy woollen jumper, his thick mop of dark hair regrown after being shaved off for the role. To his surprise, the head shaving proved to be the hardest part.

“I thought it was going to be this innocuous part of the whole process because you cut your hair on every job,” he says. “And I was naive; it inevitably holds this poignancy because of why people’s heads were being shaved and the dehumanisation aspect to it.”

In the show, his eyes are pink-rimmed, his complexion washed-out and he looks drawn, having lost weight for the part (although his dimples are omnipresent). But he’s hesitant to discuss the weight loss. “Because sometimes it can be seen as an acting exercise, though this was just a case of Tali [Shalom-Ezer, the Israeli director] and I having honest conversations about wanting the story to feel believable and compelling on screen. It was a necessary part of it.”

This is a role for which he prepared more than any before “by quite some way”. And his first move was to board a plane to Krakow.

“Not only to pay my respects, but also to see the camp through Lale’s lens,” he says. “So much of it is about his experience there and it felt important. You bear witness to the scale and the scope of it.”

It was Hauer-King’s second time at Auschwitz, having visited as a young teenager when he was the only Jewish boy on a school history trip. “But there was a Jewish teacher, and I remember having this really bonding experience with him. Going 14, 15 years later, you experience it differently, because you know more about the history. But the thing that was the same was this overpowering sense of weight and darkness.”

Did he take someone for support? “I was going to go on my own, which was a completely mental decision, because you really want to have someone to hug and to cry, and to talk or not talk. Luckily, I saw sense and asked my friend to come with me. I was very grateful to have him there.”

Hauer-King says his many months of preparation for the role involved delving into history, watching documentaries and reading survivors’ accounts, and more specific research for this story. He worked with historical and cultural adviser Naomi Gryn, daughter of Holocaust survivor Hugo, who equipped the cast members with resources. And he spent time watching testimony given by Lale in the 1990s, and later to Heather Morris, the author of the 2018 bestselling book upon which this series is based. Hauer-King talked to Morris herself, too, to get to know Sokolov and his spirit, kindness and charisma. “She was so insistent that we knew about how cheeky and flirtatious he was, and I loved hearing that.”

Telling the story of any life that has been lived is already a challenge, and Jonah-King felt “overwhelmed”. “Because where do you start? The experiences of Lale feel insurmountable. Getting my head around that was a big challenge.”

It was all about the research. He also spoke to Lale’s son Gary, Lale and Gita’s only child who lives in Australia where Lale died in 2006 aged 90. Gary has flown over to attend the series’ screenings; they met earlier today.

“You have to be incredibly sensitive, but to feel like he was pleased with how we had portrayed that story, and for him to then decide to fly over and spend time with us, has been the most meaningful part. He came in this morning with his two girls, and we all immediately burst into tears.”

Today Hauer-King is circumspect on criticism surrounding the book’s factual inaccuracies, proclaiming his admiration for Morris for telling Lale’s story so respectfully, and how its huge success opened the opportunity to make the series. “But what’s great about then adapting it is that you can build on the work that she’s done,” he says. “You can show more as a TV show.”

The book’s implication that there was only one tattooist in Auschwitz they were able to fix in a single frame showing several side by side. The creative team recreated a large portion of the camp just outside Bratislava, and the actor says that filming there 12 hours a day it was “impossible for it not to impact you on a very profound level… It’s pretty eerie”.

It’s something he’s clearly given a lot of thought to, although he’s wary of sounding “indulgent” when discussing the challenges of such a weighty role.

“It was hard finding the balance between wanting so much to be professional and to not let yourself get caught up in some kind of indulgent way where you’re thinking about how difficult it is to be making the show given the privilege of going home every night; and yet, if you didn’t feel impacted on a profound level, and if you weren’t getting extraordinarily upset at times, you wouldn’t be doing the job properly.”

He adds: “Your mind and your body play tricks on you, they really do, so no matter how much you tell yourself you’re here to do a job and the respectful thing is to not get swept up, it’s impossible not to. So it’s trying to allow both of those to coexist.”

For a release from the heaviness of the job, he turned to music, and had a bath every night and watched TV, namely Yellowstone and the very-much-not-light-hearted series The Last of Us (“it felt like an escape”). And as a self-proclaimed avid Arsenal fan, he watched as much football as he could.

“It felt like a ridiculously different headspace. This whole job was about finding balance. Sometimes jobs are stressful and there can be a lovely social aspect, particularly when you’re away. On this one, I didn’t have the energy, so I kept myself to myself.” But mainly, it was about switching off through music. He created a meditative playlist for Lale, featuring the kind of music that he might listen to today; track after track of Hans Zimmer. His face brightens as he explains this was long before he knew the great composer would be composing the score for the series. “It was a full-circle moment, listening to beautiful piano renditions of Cornfield Chase, and then when I was told that he was going to be doing this, obviously I was like, ‘Look at my list! I’ve wished this into existence.’”

London-based Hauer-King was born to Jewish parents, the American psychotherapist Debra Hauer and London restaurateur Jeremy King, and attended Eton before reading theology at Cambridge. While the casting of non-Jews in Jewish roles can feed into Jewish stereotypes, the lead roles in this series are played by Jewish actors, including Hollywood legend Harvey Keitel as the older Lale.

Hauer-King says it was “wonderful” having so many Jewish creatives involved. “You just knew the story would be looked after and done respectfully. Casting authenticity is such a complex conversation. If there’s one thing that we’ve all collectively learnt, it’s that telling a story about a specific lived experience needs to have people with some degree of that lived experience telling the story. And that can be behind camera, or in front of camera.” He ponders on the very “real thing” that is inherited trauma, and how the way stories are passed on make us who we are and can trigger something within us. “There’s no question that it informs me as a person, and probably informed my approach to this and my role.”

Recalling his mother’s best friend — Vera, a child survivor of Bergen-Belsen — he says this time in history couldn’t possibly feel new to him. “It’s a big part of probably any Jew’s growing up because you’re always aware of your own connections to it, or at least by one or two degrees of separation.” But what struck him most of all was Vera’s kindness and compassion. “For her to be that way, having had the experience that she had, is an extraordinary thing. And that I see in Lale.”