Ealing Studios, planning a bank robbery movie, asked the Bank of England to devise a way in which a million pounds sterling 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 this movie.
Audrey Hepburn (Chiquita) was considered for a larger role in this movie, but stage work made her unavailable. Sir 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 movie.
"Of all sad words of tongue or pen, the saddest are these - it might have been" is a taken from "Maud Muller", a poem by John Greenleaf Whittier.
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.
Sir Alec Guinness was paid six thousand pounds sterling. His regular salary at this time was twenty-five thousand pounds sterling.