Piotr Glowacki reduced his weight from 78 kg to 60 kg for the role and was learning boxing for a year. He also participated in other physical activities during this year, so his body would resemble that of an ex-boxer in Auschwitz.
The film is based on a true story of Tadeusz "Teddy" Pietrzykowski, a prisoner of Auschwitz concentration camp. While incarcerated there, he engaged in around 40-60 boxing fights, losing only once - to Dutch middleweight champion Leen Sanders.
The prisoner number of Tadeusz "Teddy" Pietrzykowski in Auschwitz was really "77". He arrived there on June 14, 1940 on the first transport of prisoners.
The film was filmed in Poland. Outside scenes were filmed in a reconstructed (for the film) Auschwitz concentration camp near Piaseczno. Most of the set consisted of two large buildings constructed especially for the film, which where digitally replicated in post-production. Inside scenes were filmed in Modlin Fortress in Nowy Dwor Mazowiecki and other undisclosed locations.
For language dubbing (synchronization) they used an AI-based software that fitted the voice to the lips/face.