Script versus novel
The adaptation really stays remarkably close to its source material, especially considering the fact that this is not just a short story but a full-length novel. Exton makes only minor changes. First, all references to Hastings's Argentinean farm, Poirot's retirement (and declining fame) and Poirot's investigation on the Blue Train have obviously been removed, since these events still hadn't occurred in the television series. Second, Miss Lemon is added to the investigation (she investigates the doctor in Harley Street and very sensibly takes Hastings's place in the seance), and Japp's role is somewhat expanded (he is present in the novel, but here he becomes the investigating officer, replacing Colonel Weston. Also, Hastings gets to visit Seton's solicitor in London on his own. Third, certain plot elements are dropped, like the red herring of Lazarus's offer for Nick's painting, Poirot's A-J list of suspects, Maggie Buckley's letter to her parents and the face (later revealed to belong to Freddie's husband) outside the hotel window. Neither of these clues are essential to the plot (or the reveal), so it makes sense to remove them - probably because of time constraints. Finally, Hastings and Miss Lemon are not told that Nick is still alive (which makes sense - not just because the viewer would be just as shocked as the suspects that she is - but also because Hastings would never be able to keep a secret, as Poirot has pointed out more than once). All in all, this is a wonderfully crafted adaptation that manages to stay true to the source material, and possibly even improving on it in certain places.
Directing, production design, locations, soundtrack
Renny Rye makes excellent use of the location - there's an added opening scene with a plane approaching the coast (a nice way to include the novel's opening remarks by Hastings on how beautiful the Cornish coast is). The production design in this episode is also great, with a exquisite "Majestic Hotel" set and some beautiful seaside locations. The episode was shot in Salcombe, South Devon, and Salcombe Yacht Club doubled as the nursing home (see link). Moult House, near South Sands, in South Devon, became "End House", and the end scene was shot at the beach. Finally, Kingswear Railway Station became "St. Looe Train Station" (see link). The soundtrack to the episode is mostly appropriate, but it has sadly not been released. (The score was composed by Fiachra Trench, who stepped in through most of Series Two as Gunning was unavailable).
Actors and characters
In terms of character development, Hastings gets to display his love of cars and golf once more. Poirot smokes his Russian cigarettes again, we get to know about the necessity of eggs of exactly the same size (referred to in the novel in a conversation with Nick Buckley), and he gets to sit in his room and think. Miss Lemon gets to use her detective skills again - and we see a hint of her interest in the occult (even if it's imposed on her by Poirot).
There are so many great performances in such a long film, but obviously, Polly Walker (Nick Buckley) is the one that really stands out. The contrast between the charming girl we encounter in the beginning of the episode and the angry and downright evil woman we see in the end scenes is very well done!