21. The formidable Mrs. Dane Calthrop, a vicar's wife, is one of only a handful of characters who appear in both a Christie series (meaning Poirot/Miss Marple) and a standalone novel. Which are the two books in which she appears?
From Quiz Glimpsed Again: Recurring Supporting Characters
Answer:
"The Moving Finger" and "The Pale Horse"
"The Pale Horse" is unique insofar that there are four characters in it who previously appeared in a Christie series: Mrs. Dane Calthrop, who met Miss Marple in "The Moving Finger"; Rhoda and Major Despard, who worked with Poirot in "Cards on the Table"; and Mrs. Ariadne Oliver in one of her two non-Poirot books (the other being "Parker Pyne Investigates").
Mrs. Dane Calthrop is described as quite an original person whose trains of thought are sometimes hard to follow. But she inspires trust and can provide unusual insights. Both Jerry Burton (in "The Moving Finger") and Mark Easterbrook (in "The Pale Horse") turn to her for help at some point.
Other Christie characters who appear both in a series and in standalone books include Superintendent Battle ("The Secret of Chimneys", "The Seven Dials Mystery", "Murder Is Easy", "Toward Zero", and "Cards on the Table" with Poirot); Colonel Johnny Race ("The Man in the Brown Suit", "Sparkling Cynanide", and twice with Poirot in "Death on the Nile" and "Cards on the Table); and Miss Felicity Lemon (who worked for Parker Pyne before she became Poirot's secretary). The sleepy Colonel Pikeaway, a high-ranking secret service agent, takes it even one step further: he appears in a Poirot novel ("Cat Among the Pigeons"), a Miss Marple novel ("At Bertram's Hotel"), a Tommy and Tuppence novel ("Postern of Fate"), and a standalone novel ("Passenger to Frankfurt"), often accompanied by his trusted colleague, the enigmatic Mr. Robinson (who is described in "Cat Among the Pigeons" as not having the looks of a "Mr. Robinson" at all).