The movie is based on a play by German playwright Rolf Hochhuth, which started a lot of heated discussions and arguments after its first release in 1963. It even caused some diplomatic tensions.
The Vatican did not give permission to shoot the film in its buildings. After searching for a location of adequate size for the scenes taking place in the Vatican, Costa-Gavras finally chose Europe's largest building, the House of the People (Casa Poporului) in Bucharest, Romania. Some of the outdoor scenes were shot in Mogosoaia Palace, approximately 14 kilometers northwest of Bucharest.
The car arriving behind the fence after 12 minutes into the film is a 1948 Ford Pilot V8, and is too new to be featured under the WW2.The film also contains a Mercedes-Benz 170S, which is a 1949 model. The film is supposed to show WW2, several years earlier.
In one of the scenes they say that the Treblinka camp is out of gas, referring to Zyklon B. Treblinka didn't use Zyklon B, instead they used carbon monoxide.
Stephan Lux: [interrupting a session of the Assembly of the League of Nations, Geneve, 1936]
My name is Stephan Lux. I am Jewish. The Jews are being persecuted in Germany and the world doesn't care.