Ealing Studios, planning a bank-robbery film, asked the Bank of England to devise a way in which a million pounds could be stolen from the bank. A special committee was created to come up with an idea, and their plan is the one used in the film.
Arriving in Paris, Pendlebury recites the words, "Gay, sprightly land of mirth and social ease"; Holland later repeats the phrase in reference to Rio de Janeiro. This line is a subtle reference to the movie's plot, because those words come originally from the 1765 poem "The Traveller" by Oliver Goldsmith.
Audrey Hepburn was considered for a larger role in this film, but stage work made her unavailable. Alec Guinness was impressed with the young actress and arranged for her to appear in a bit part. This is considered to be Hepburn's first appearance in a major film.
The trivia item below may give away important plot points.
One of the rationales the filmmakers considered in not allowing the mob to get away with their crime is that prevailing censorship criteria in the U.S. would have cost them the lucrative American market. (The U.K. had no such rule, and the filmmakers could have had the criminals successful if desired.)