Miss Marple's friend Mr. Stringer is played by Margaret Rutherford's husband Stringer Davis.
Even more than that, a few references in Murder Most Foul suggest that Mr. Stringer's first name is James. As was the first name of the real-life Stringer Davis ("Stringer Davis" being a stage name).
Disowned Adaptation: Downplayed. Though Christie voiced dissatisfaction with the films due to their plot changes and Denser and Wackier tone, she did still dedicate the novel, The Mirror Crack'd From Side to Side to Margaret Rutherford "in admiration". They got on quite well in real life.
Reality Subtext: In Murder Most Foul, the villain appears to have some hereditary violent insanity. Margaret Rutherford herself was terrified that this would happen to her, as her father spent a great deal of time in asylums after murdering his own father and her mother ended up committing suicide while pregnant.
