Script versus novel
Agatha Christie always considered this one of her lesser efforts. It's basically just an expansion of the short story, 'The Plymouth Express', and it consists of a wide range of incredible plot points. Therefore, it was no surprise that Guy Andrews felt the need to radically rewrite the story (although, like with his other adaptations, he adds some equally ludicrous points himself...). It's almost impossible to sum up all the changes, but I'll try to outline the most important ones. First, he makes everyone travel on the same train to Nice (cf. MOTOE), including the count, Lady Tamplin, Corky ("Chubby" in the novel), Lennox, Mirelle Milesi (just Mirelle in the novel) - as well as the ones from the novel (Ruth, Katherine, the maid etc.). Second, Poirot appears much earlier than in the novel, and he is well acquainted both with Katherine and Ruth before the train journey. In fact, he becomes Katherine's "avuncular" (with a particularly charming introductory scene at the hotel). Third, there is an elaborate birthday party for Ruth in London before the train journey, attended by everyone involved, and a party at Lady Tamplin's once they arrive in France. Fourth, Ruth asks Katherine to change compartments on the train, suggesting that Katherine might have been the intended victim. Fifth, there's an entire new back story to Katherine (her father killed himself after Van Aldin Oil bought his company and fired all his employees). Similarly, an entire back story is added to Ruth's mother, who is revealed to be in a convent / convalescent home in France, where she has become a nun. Sixth, Mirelle is Rufus Van Aldin's lover and not Derek Kettering's. Seventh, the extremely complicated background with "the Marquis", Demetrius and Zia Papopolous is removed (a wise decision). Instead, Corky finds the imitation ruby, Derek is in heavy gambling debt to the Count, and one of the culprits attempts to kill Katherine in her sleep. At the end of the film, the murderer commits suicide, instead of just being arrested by the French police as in the novel.The end result is such that, if you can get past the changes, the adaptation isn't half bad. I particularly like the way in which Poirot's avuncular qualities are brought to the fore. I'm not going to defend the changes (what's up with Andrews and nuns? He adds them in all of his three adaptations!), but I don't really mind him making them, both because the novel itself hardly can be considered plausible and because there was a need to distinguish it clearly from 'The Plymouth Express'.
Directing, production design, locations, soundtrack
Hettie Macdonald's direction has divided opinions, I have noticed. Her experimental use of camera angles and shadows takes a bit of getting used to, I admit, but I find them mainly effective. Also, she benefits greatly from some really nice locations and excellent production design. The locations include (Sheraton) Park Lane Hotel, London, Menton Old Town, La Rotonde, Beaulieu-sur-Mer, Villa Maria Serena, Menton, Wansford Station and Nene Valley Railway, Musée des Beaux-Arts - Palais Carnolès (Nice railway station) and Freemason's Hall London (party scenes).Stephen McKeon, who composed the music for Series Ten and Eleven, does a nice job with this particular episode. See his website here. Also, if you can find the 45 minutes behind-the-scenes documentary on Series Ten, there is plenty of more information on this production there, including a short interview with scriptwriter Guy Andrews.
Characters and actors
Again, Suchet adds little touches of Poirot's sense of loneliness (see, for instance, the end scene). It's lovely to see the character develop from the early years to his retirement. Of the guest actors, most suit their roles, but Lindsay Duncan and Georgina Rylance are the standouts in my opinion.