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.
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.
In the car chase scene at the end of the film, an officer uses a police box to report seeing a police car being driven by a man in a top hat. In fact, the driver is wearing the uniform of the police as originally set up in 1829 by Sir Robert Peel , known as "Bobbies" or "Peelers."
Screenwriter Clarke is said to have come up with the idea of a clerk robbing his own bank while doing research for the film Pool of London (1951), a crime thriller surrounding a jewel theft. He consulted the Bank of England on the project and it set up a special committee to advise on how best the robbery could take place.
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.)