Though Hartmann and Legat are fictional, Harris confirms that they are partially inspired by the diplomat Adam von Trott zu Solz and the scholar AL Rowse. The latter, who was gay, wrote about his intense platonic attachment to Trott at Oxford. Trott, though a considerably less amiable character than the fictional Hartmann, went on to join Claus von Stauffenberg's 1944 plot to assassinate Hitler. It failed and Trott was executed.
The document that Helen shows Paul at her apartment is the Hossbach Protocol, Hitler's secret decree to his generals for the creation of the Lebensraum (Vital Space) of Germany. So called for Hitler's military adjutant, Colonel Friedrich Hossbach, the Hossbach Protocol was evidence of Hitler's intention for a European war. When the war started, the protocol was developed into the Generalplan Ost, or Master Plan for the East, a program for ethnic cleansing and genocide that planned the deportation 31 million people eastwards, with Siberia as the final destination. Then, the population would be systematically exterminated through starvation and forced labor, and their territories colonized with 8-10 ethnic Germans.
Paul is warned not to smell like cigarette smoke in the presence of Hitler. This is based on the fact that real Hitler hated smoking and absolutely banned everyone for smoking in his presence.
The actor Ulrich Matthes, who plays Adolf Hitler in this film, previously played Nazi propaganda minister Dr Joseph Goebbels in the production La caída (2004).