Script versus novel
Gatiss's script manages to transport the plot from its 1960s setting to the series' 1930s setting almost seamlessly, with the odd fact that Halloween parties in the UK probably were far less common in the 30s. For instance, he removes any references to LSD and other drugs. As to changes to the plot, he removes a couple of characters, most notably Superintendent Spence and his sister Elspeth McKay, but also Ann Reynolds, Dr Ferguson, Harriet Leaman (the cleaner), Miss Emlyn, and the boys, Nicholas Ransom and Desmond Holland. It would be nice to have Spence included, particularly since he was present in the two previous novels he was in (Taken at the Flood and Mrs McGinty's Dead), but I suppose he felt there would be too many investigators on the case (Poirot, Mrs Oliver, the local police and a retired Spence). The other characters are minor and consequently their removal matters little to the plot. Gatiss adds a couple of new characters, i.e. two grown-up children for Mrs Drake (Edmund and Frances). He makes Mrs Reynolds a step-mother to her children. Also, Mrs Goodbody (the local 'witch') fittingly replaces Elspeth as the local gossip from whom Poirot gets his information on the suspicious deaths (one suspicious death, Charlotte Benfield, is removed). Moreover, the Jane White (Beatrice in the adaptation) death is given a somewhat different backstory (or, perhaps I should say, a more outspoken one). The character of Ambrose is deleted, so Beatrice White is revealed to have been in love with Miss Whittaker (who is a church organist, not a teacher here). Some viewers have reacted to the more upfront display of homosexuality, but I think it's been beautifully done in a very touching scene by the lake where she drowned (as an aside, this is the only Christie novel in which the word 'lesbian' is used). Furthermore, Lesley Ferrier was seeing Frances Drake rather than Nora Ambrose. Also, the reverend gets a more central role, with an au pair scheme to help girls like Olga (making him a potential suspect). Of minor changes, Mrs Oliver is down with a flu for most of the episode (I'm not sure if that's supposed to add comedy to the proceedings or if it was a result of Zoë Wanamaker's availability), Poirot arrives on the train together with Michael Garfield (who has just arrived from Greece), Poirot doesn't stay at a guest house but at Mrs Butler's house, Mrs Drake's husband was killed by what seems like a hit-and-run accident, and several horror and spookiness references are added, e.g. to Edgar Allan Poe and Matthew Hopkins (probably a result of Gatiss being a big horror fan). Poirot, obviously, dislikes the tradition of reading horror stories around Halloween (he prefers remembering the dead).All in all, Gatiss's adaptation works well, and it's a more or less faithful retelling of the novel.
Directing, production design, locations, soundtrack
Palmer's direction suits the episode. He has emphasised the darkness and autumn colours of the season. Also, I particularly like the use of the snap-dragon game, which I assume is partly Gatiss's and partly Palmer's idea. This brings to mind other adaptations that revolve around games and rhymes, like One, Two, Buckle My Shoe and 'How Does Your Garden Grow?'. The production design also suits the episode, again with an emphasis on the season. The main location is a private estate in Oxfordshire. The soundtrack works particularly well for the episode. Composer Christian Henson emphasises the snap-dragon game in collaboration with Palmer and Gatiss.
Characters and actors
It's always a joy to see David Suchet and Zoë Wanamaker together as Poirot and Mrs Oliver. They really have excellent screen chemistry. A shame that Mrs Oliver is bedridden for most of the episode, though. Poirot is in investigation mode in this story, even if faced with opposition from the local police. Also, it's good to see George back in the fold, too. Of the other actors, Deborah Findlay and Julian Rhind-Tutt both stand out, but I think particular credit should go to Mary Higgins, who plays Miranda.