Alec Guinness nearly drowned in the scene where The Admiral goes down with his sinking ship. Guinness was held down by wires while the set filled up with water. Once the scene was wrapped, the crew started to leave until one technician suddenly realised that they had forgotten to release the actor 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.
The scene where six members of the D'Ascoynes family, all played by 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.
The right of peers to be tried in the House of Lords was abolished in 1949, the same year the film was released. The two were not connected, the right was abolished due to a combination of a Labour Government and reaction from a drunk driving case where the lordly defendant was tried in the House of Lords.
Agatha's death in the film caused some consternation for Alec Guinness. The scene in question - a hot air ballooning accident filmed in a field next door to Pinewood Studios - prompted him to ask the producers if he was well insured. They told him that he was, to the tune of £10,000, but Guinness didn't think that was enough. He then declared that the balloon could not be raised any more than 15 feet unless they raised the insurance to £50,000. Ealing Studios was renowned for being very penny-pinching and it naturally refused Guinness' demand, pointing out that he would be accompanied in the balloon by a well-qualified Belgian balloonist hidden in the basket with him. Guinness was undeterred in his refusal to perform the stunt, so the scene in the finished film is not him but the Belgian balloonist wearing Agatha's dress and wig. Guinness had the last laugh, however, when a high wind pulled the balloon off course. The Belgian balloonist was found 50 miles away, having had to pitch into a river.
Michael Balcon was known to have said to director Robert Hamer, "You are trying to sell that most unsaleable commodity to the British - irony. Good luck to you." It worked, of course; the film was a considerable success upon release.
An alternate ending was required for the US, where distributors balked at the film's ambiguous ending (The US 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, where they had been quietly filed away in a film storage facility.
The original camera negative was saved from the Henderson's Film Laboratories fire of 1993, just before a massive nitrate explosion destroyed the negatives of many other films including several other Ealing comedies.
Although Ealing boss Michael Balcon later professed that this was his favorite of the Ealing films he produced, at the time of production he was less favorably inclined towards it, to the extent that he refused director Robert Hamer the chance to follow it up with his long-cherished project set in the West Indies. Hamer ultimately only directed one more film for Ealing, His Excellency (1952).
The opening music to the film is based on the aria "Il mio tesoro" from Don Giovanni by Mozart. It appears several times in the film. For instance, Louis' mother plays it on the piano, and his father sings it near the start of the film.
The trivia item below may give away important plot points.
Contrary to popular rumor, Dennis Price's character actually only killed six of the eight Alec Guinness characters. The second "victim," the bank manager, dies of a stroke or heart attack; Price had grown fond of him, and even says at one point he was glad he didn't have to kill him. Another Guinness character, Admiral Horatio D'Ascoyne, goes down with his ship when the vessel collides with another, and is only seen saluting as the water rises over his head. This collision is possibly a reference to the real-life disaster which involved the battleships HMS Victoria and HMS Camperdown in 1893.