Also helping Bruno Ganz in preparing for the role was the unique, only known recording of Adolf Hitler when he held a private conversation with Field Marshal Gustaf Mannerheim of Finland; at that time, he was a World War II ally of Germany against the Soviet Union. Hitler unexpectedly showed up to congratulate Mannerheim on his 75th birthday on June 4, 1942. Finnish intelligence agents secretly made the recording in a train wagon, as Hitler did not allow recordings nor photographs to be taken in private. Some eleven minutes of the recording feature relaxed, normal-tone talk in which Hitler generally describe his views about the war. One of two copies of the tape was discovered in 1992 and has since been studied by scientists and historians.