In his first meeting with Charles, Eustace makes reference to there being "a cat among the pigeons" within the mansion. This is an insider reference to another Agatha Christie work, filmed for TV, Agatha Christie's Poirot: Cat Among the Pigeons (2008). Amanda Abbington (Clemency) played a role in that adaptation, as well.
Lady Edith says to Charles, "Sophia believes there's a Borgia living under this roof." The Borgias were a powerful family during the 15th and 16th centuries who were suspected of many crimes, notably murder by poisoning. Max Irons's (Charles Hayward) father Jeremy Irons played Rodrigo Borgia (aka Pope Alexander VI) in The Borgias (2011).
The location used for the foyer and staircase of Three Gables is Wrotham Park in Hertfordshire. The same location was used for the film Gosford Park, written by Julian Fellowes, who also wrote the screen adaptation for Crooked House.
Julian Sands, Christian McKay, and Amanda Abbington are all "Agatha Christie veterans" and had previously appeared in TV renditions of Hercule Poirot and Miss Marple novels before working on this Christie film adaptation.
Josephine tells Charles that she is reading "Salome" and gives him a short synopsis of the story. Glenn Close (Lady Edith) famously played Norma Desmond in the musical version of "Sunset Boulevard" - twice - in which Norma has written an overly long screenplay adaptation of "Salome".
There are 4 major plot-points in this film adaptation that stray from the original Agatha Christie source novel. 1) Charles and Sophia are actually engaged and not simply ex-lovers from Cairo; Charles is more personally involved from the start. 2) The method of attempted murder of the young lady switches from a blunt instrument attack in the novel to a severed rope ladder in the film. 3) In the denouement, the older female leaves two notes instead of just one. She wished the detective to know the full truth. 4) The detective and his lover are not present and/or involved in any car chase sequence as shown in the finale of the film.