In unwavering commitment to his role in László Nemes’ harrowing competition title Moulin, up-and-comer Félix Lefebvre slept in the grizzly, mattress-less cell he and co-star Gilles Lellouche were shooting in — for a good few nights.

“There was bats. I didn’t have any idea what time it was. And I felt like those guys during the Second World War, [who] actually went through it and were having such a hard time,” the young French star tells The Hollywood Reporter. “I just wanted to be the best version of me, as an actor.”

To provide some much-needed context, the Hungarian filmmaker returns to the Croisette — where he picked up the Grand Prix in 2015 for Son of Saul, the film that would go on to claim him an Oscar — with the real-life story of French resistance hero Jean Moulin (played by Lellouche). It follows Moulin’s arrest in June 1943 as he attempted to reunify the forces of the Secret Army, and wound up tortured by the sinister Klaus Barbie, head of the Gestapo (Lars Eidinger).

To those unfamiliar with French World War II history, the figure might not even ring any bells. This was not the case for Lefebvre, who grew up studying Moulin. “It’s in our history class program. We hear everything about him — he’s this heroic figure, a leader of the resistance in France during the Second World War. It’s one of the things that you learn at school that you actually remember,” the 26-year-old continues, “because you hear that this guy went through the worst torture and didn’t say a word. So as a kid, you hear that, and you start thinking, ‘Well, if I went through a lot of pain, would I be brave?’ I have a very vivid memory of [learning about] this man.”