Trivia: Sidney Lumet's first choice for Hercule Poirot was Alec Guiness, with Paul Scofield as his second choice. Because they both were busy with other projects, Albert Finney was chosen, even though Lumet felt he was too young for the role.
Trivia: The final repetition of the line "He'd kill us if he had the chance" had its inflection changed because preview audiences became preoccupied with questions over whether Harry had misinterpreted the couple's conversation or if they were deliberately being misleading (or both). The ambiguity was initially intended, but appeared to work a little too well and distracted the audience from appreciating the rest of the ending, so this delivery of the line was made more definitive.
Trivia: The voice of Mr. U.N. Owen on the tape is that of Orson Welles.