Dick Clement later said Kirk Douglas requested a meeting before filming started. "Being an American, Douglas got straight to the point. He said I can't play comedy. And do you know what - he couldn't."
There was a discarded idea for a spectacular ending to the boat chase put forward by the director whereby either the British Gunboat or the East German boat would explode, but the idea never reached fruition, as neither owner would agree to sell their craft.
For its French release, Marlene Jobert (Fabienne) was billed above Kirk Douglas (Andrej).
Kirk Douglas is most oddly cast as a Bulgarian. However, he doesn't attempt the accent, and speaks in his normal American voice. This is not explained.
Tom Courtenay (who was not yet Sir Tom Courtenay) had returned to film after a two year absence: In 1969 he starred in the same director's other spy comedy, Otley, and this would be the last theatrical motion picture Courtenay appeared in until The Dresser in 1983. He worked mainly on stage, and a few television movies.