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.
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.
Many of the scenes filmed on location in London show that the city was still being rebuilt in 1951, ten years after the Blitz.
Sir Alec Guinness was paid six thousand pounds sterling. His regular salary at this time was twenty-five thousand pounds sterling.
"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.