Script versus novel
As is almost always the case with Exton, the script remains largely faithful to its source material, with only some minor and largely understandable changes to the main plot. To begin with, he adds an (almost necessary) prologue in South Africa, set in 1896, in which we see Simeon Lee kill his partner Ebeneezer (not in the novel) and being rescued by a woman, Stella, in the desert (not in the novel), who later turns out to be the mother of one of the illegitimate children mentioned in the novel. Moreover, he adds some scenes in which Poirot and Japp (who replaces the local Colonel Johnson in the novel) prepare for Christmas - and a subplot concerning the presents the two give to each other (this change in fact becomes an important plot point, as it leads to Poirot's visit to a village store in which he sees the pig balloons and the fake moustache). Third, the character of Stephen Farr is deleted (he was no more than a red herring), and instead Pilar's romantic sentiments are directed towards Harry Lee. Fourth, the playing of the 'Dead March' on the piano is deleted and replaced with a dining room quarrel. Fifth, the Lady Macbeth quote is removed (probably because the crime scene is much less blood-filled (to avoid PG rating, possibly?)). Other minor changes include the discovery of the diamond case in George Lee's room (planted there by Sugden, we are told), and the fact that Stella from the opening sequence is Sugden's mother. Generally speaking, however, the script is faithful to its source. Some of the interviews are shortened and several sections are moved around, but essentially the story is very recognisable.
Directing, production design, locations, soundtrack
Edward Bennett's directing is competent, and he manages to convey the proper Christmas atmosphere (which is quite an achievement - the episode was shot in April!). The colour grading is noticeable here, too, since the light is particularly harsh and white (reflective of the wintery setting, I suppose). The locations used include Chilham Manor, Kent (the Lee family home), Chilham village (the pub and Sugden's house), Chilham Church. See this link for photos. Christopher Gunning's soundtrack is as good as ever, with a nice seasonal touch to it.
Characters and actors
Suchet gets to display many of Poirot's character traits, like his love of (Belgian) chocolate (also seen in 'The Theft of the Royal Ruby'), his concern for the central heating and general dislike of cold weather and cold manor houses (see, for instance, 'The Mystery of Hunter's Lodge'). The scenes with him at the cold station are reminiscent of the Murder on the Orient Express adaptation, too. Of the guest actors, Vernon Dobtcheff does a particularly exceptional job as the dislikeable family patriarch.