For its French release, Marlene Jobert (Fabienne) was billed above Kirk Douglas (Andrej).
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."
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.
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.
While Kirk Douglas had reservations about doing comedy, he did star in 20,000 Leagues Beneath The Sea which has many comedic elements, particularly his free-spirited and often jokey character.