This would be Geraldine McEwan final time as Miss Marple. Julia Mackenzie would replace her in the last three series.
This was Herbert Lom's final acting project before he died on September 27, 2012.
In the scene just before the sleepwalker, Miss Marple is reading "Not Quite Dead Enough" by Rex Stout, a real novel written about the fictional detective, Nero Wolfe.
Actor Herbert Lom provides the voice of Jason Rafiel in the audio message to Miss Marple; he had previously appeared on camera as Professor Dufosse in "Marple: The Murder at the Vicarage."
In the "Miss Marple" series, Miss Marple meets "Jason Rafiel" in the 1964 novel "A Caribbean Mystery" which isn't seen until series 6 of "Agatha Christie's Marple" with Julia McKenzie as "Miss Marple". As was done in the Joan Hickson "Miss Marple" series, "Nemesis" was filmed prior to "A Caribbean Mystery".