In his autobiography, cinematographer Oswald Morris recalled how John Huston showed very little interest or enthusiasm for directing this movie and would arrive late on-set, largely unprepared for the day's schedule. It often was left to Morris and the crew to fill the gap and set up the shots for the day for when Huston eventually arrived and also to help Paul Newman, who also was disappointed by Huston's attitude.
Walter Hill got sole credit for the screenplay but later claimed that he had only written the first half of the movie. Other people known to have worked on the script at various points include playwright William Fairchild, author Gerald Hanley, historian Alan Moorehead, and producer and director John Huston's long-time collaborator and assistant, Gladys Hill, as well as Huston himself. According to Huston's autobiography, the script was still being written in the last days of filming. In that book, he felt the need to "apologize" for this movie.
This movie and its source novel were loosely based on the identification, defection, and escape from Wormwood Scrubs Prison in 1966 of Russian spy George Blake (a.k.a. George Behar) who was working in British intelligence as a double Agent but was not one of the Cambridge Five spies.
During a break in filming, James Mason took a evening stroll around Dublin, Ireland. He noticed that he was being followed by a woman, obviously someone who recognized him. Eventually she approached him and asked "Excuse me sir, but would you be Mr. James Mason in his later years?"
For a key action scene, producer and director John Huston burnt an old manor house near Galway for a second time. It previously had been gutted during the Irish Civil War around fifty years earlier.
Leo Genn: a lawyer in the trial scene, He was a real-life barrister who helped prosecute the Belsen war crimes trial.