The film
- Acting in the Dark: Billy Wilder didn't give the last 10 pages of the script to the cast until the time came to shoot those scenes, so they wouldn't be able to give away the secret.
- Approval of God: When the film was released, Agatha Christie said it was the only movie based on one of her stories she had actually liked. Later, after Murder on the Orient Express (1974) was filmed, she said she liked that one too.
- Reality Subtext: Marlene Dietrich's characterization of a woman desperately in love was also enhanced by her real-life crush on Tyrone Power during shooting. According to his biographer, the actor was embarrassed by her advances.
- Romance on the Set: According to Billy Wilder, Charles Laughton had a crush on Tyrone Power. Marlene Dietrich fancied him, too.
- What Could Have Been:
- William Holden was the first choice for Leonard, but he was unavailable. Gene Kelly, Kirk Douglas, Glenn Ford, Jack Lemmon, and even Roger Moore were considered for the part.
- Ava Gardner and Rita Hayworth were also considered for the role of Christine Helm.
