Script versus novel
Nick Dear does an excellent job with the adaptation of this particular novel. The script has the perfect balance between melancholy, humour, seriousness and Poirot-ness. Dear deletes the character of David Angkatell (and he's not missed). He adds an opening scene with Henrietta Savernake and John Christow (which makes perfect sense) and some arrival scenes for Poirot (delightful). Also, Poirot gets invited to dinner the evening before the lunch, a small change which, again, makes sense, and the murder weapon isn't discovered outside his cottage (thankfully - that never made sense to me). Moreover, Poirot gets the horse clue right not because it's like the Horse of Troy, but because the person in question explains that she hates horses (a much more satisfying reason, if you ask me, and very much in keeping with Poirot's sense of psychology). Dear also removes Edward's attempted suicide, compresses the scenes with Edward and Midge in London (but they are referred to in the dialogue), removes Henrietta's visit to Mrs Crabtree, and changes Gerda's death (SPOILER she goes to John's study and takes potassium cyanide). All in all, though, the adaptation is remarkably faithful and beautifully written, possibly one of the best episodes of the entire series. I'm always delighted when Christie's stories are taken seriously.
Directing, production design, locations, soundtrack
Langton's direction is a joy to watch, especially the opening sequence and the autumnal feel to every shot. I particularly like the staged shots of the pool, perfectly recreating the sense of artificiality / theatricality Poirot comments on in the novel. The locations used for the episode are perfect, too. Duchess Street in London was used for Christow's Harley Street home. The soundtrack is wonderful, Gunning outdoes himself. The beautiful main theme of the episode is available on his album The Film and Television Music of Christopher Gunning.
Characters and actors
Suchet's Poirot is excellent in this one. I always enjoy the cases in which Poirot's interest in psychology comes to the fore, and this is certainly one of the best on that account. Also, we have his dislike of the countryside, his breakfast (the little blobs of jam on the toast) and his sense of justice, truth, right and wrong (as displayed in the well-scripted conversation with Henrietta). The guest actors work wonderfully together as an ensemble, so it's almost impossible to pick a favourite, especially with veterans like Sarah Miles, Edward Fox and Edward Hardwicke on the cast list.