Ganz conducted four months of research and studied a recording of Hitler in private conversation with Finnish Field Marshal Carl Gustaf Emil Mannerheim in order to properly mimic Hitler's conversational voice and Austrian dialect. Ganz came to the conclusion that Hitler had Parkinson's disease, noting his observation of Hitler's shaky body movements present in the newsreel "Die Deutsche Wochenschau", and decided to visit a hospital to study patients with the disease. Ganz auditioned in the casting studio with makeup for half an hour and tested his voice for Hirschbiegel who was convinced by his performance.