The real Cicero was Elyesa Bazna, an Albanian. In his book "I Was Cicero" he retells the story, listing his collaborators as a chambermaid and his niece. The character of the countess was a Hollywood fabrication.
Director Joseph L. Mankiewicz met the real spy code-named Cicero while filming in Turkey. The man, Elyesa Bazna, offered to serve as a technical advisor on the film but was turned down. His former Nazi handler, German diplomat L.C. Moyzisch, also made the same offer to the producers but was also turned down. After the film's release, Moyzisch was very displeased by the depiction of him.
The £120,000 the Germans paid to Cicero would, after taking into consideration the exchange rates and adjusting for inflation, be equivalent to about $8,000,000 in 2014.
Ambassador Franz von Papen was a former Chancellor of Germany. After he resigned from office, he worked with the Nazis and helped get Adolf Hitler appointed as Chancellor, and was therefore instrumental in the rise of the Third Reich.
This was Joseph L. Mankiewicz's last film under his contract with Twentieth Century Fox. A great deal of preparatory work had been already been done on the film when he expressed an interest in the story, and studio chief Darryl F. Zanuck was reluctant to let him wait out the last few months of his contract without doing anything. Mankiewicz rewrote the dialogue extensively without taking a credit, but his contract was up by the time the editing process had to begin. He always claimed that Zanuck, who supervised the editing, had taken out several of the film's best scenes.