Sir Alec Guinness nearly drowned in the scene where the Admiral goes down with his sinking ship. Guinness was held down by wires whilst the set filled up with water. Once the scene was wrapped, the crew started to leave, until one technician suddenly realized that they had forgotten to release Sir Alec from the wires holding him underwater. He immediately dove into the waters with some wire-cutters and freed Guinness. Fortunately for all concerned, Guinness took great pride in his ability to hold his breath for long periods of time.
Initially, Sir Alec Guinness was only offered four of the roles. It was Guinness who insisted on playing all eight.
An alternative ending was required for the U.S., where distributors balked at the movie's ambiguous ending (the U.S. Production Code at the time stipulated that crime could not be seen to pay). These extra ten seconds were not kept by Ealing, but were unearthed at the Museum of Modern Art in New York City, where they had been quietly filed away in a film storage facility.
The scene where six members of the D'Ascoyne's family, all played by Sir Alec Guinness, are seen together, took two days to film. The camera was set on a specially built platform to minimize movement. In addition, the camera operator spent the night with the camera to ensure that nothing moved it by accident. A frame with six black matte painted optical flat glass windows was set in front of the camera, and the windows opened one at a time so each of the characters could be filmed in turn. The film was then wound back for the next character. Most of the time was spent waiting for Guinness to be made up as the next character.
Director Robert Hamer and Sir Alec Guinness got along extremely well during the shoot and formed a friendship that would last for many years to come. "Robert and I spoke the same language and laughed at the same things", said Guinness in his 1985 memoir "Blessings in Disguise". "He was finely-tuned, full of wicked glee, and was marvelous to actors, appreciative and encouraging."