This movie was made in Hungary, then a Communist country. The cast and crew were obliged to work six days a week under considerable pressure, and director John Frankenheimer was very unpopular. Sir Dirk Bogarde always referred to him thereafter as "Frankenstein", while Sir Ian Holm reported in his memoirs nearly forty years later, that Frankenheimer had, during filming, a very obvious extra-marital affair with the daughter of screenwriter Dalton Trumbo, even though his wife, Evans Evans, was in attendance.
Page ninety-six of Steve Martin's memoir "Born Standing Up", relates a story in which openly Communist Dalton Trumbo's daughter tells Martin: "He has a screenplay due in four days, and he hasn't started in yet." This is in reference to her father writing this movie at the last minute.
This movie was Sir Dirk Bogarde's biggest paycheck to date. He earned $165,000.
Director John Frankenheimer wanted Richard Burton for the part of Yakov Bok.