Adaptational Explanation Extrication: Unlike in the book, there is nothing to suggest Conchita is an impostor before it is spelled out and no explanation of how Poirot arrives at the conclusion, making the reveal of the fact seem randomly pulled off for the sake of Not Blood Siblings.
Harry Lee's rudeness is downplayed compared to the book, and his lecherous comments about Pilar are mostly replaced with genuine affection and protectiveness. The fact that during his "prodigal son" phase he didn't neglect to frequently wire his father for money is also omitted.
Magdalena Lee cheats on her husband like her novel counterpart did; however, it's never indicated she is throwing away George's money in secret from him, and she never tries to throw suspicion on Pilar/Conchita by pointing out the un-English nature of the crime.
Unlike in the book, Sugden and his mother are abandoned by Simeon Lee, while in the book he always made sure his exes and any potential children of his were financially supported. This makes Sugden and Stella much more sympathetic.
Simeon's wife is fully the innocent and sympathetic party in their terrible marriage. In the novel, the characters discuss that her The Eeyore behavior was also one of the reasons the marriage failed, not to mention that she burdened little David with her constant complaints, leading to his own psychological problems later in life.
In the book, Simeon Lee provides for his ex-lovers, while here he leaves Stella after sleeping with her, robs her, and never appears in her life again. In the books, he likewise never kills his mining partner.
As a result of the above, Superintendent Sugden's mother is intent on destroying Simeon Lee's life after he abandoned her and her child, which is never mentioned in the novel.
Ascended Extra: Superintendent Sugden's mother is The Ghost in the book, and the only thing we know is that she slept with Simeon. Here, she gets a backstory and becomes a supporting character.
Asshole Victim: Simeon Lee was a cruel, selfish young man and has grown tyrannical and petty in his old age. He is killed by his own son, and his innocent relatives don't mourn him either.
Christmas Episode: Aside from the original novel's title and setting, the adaptation also aired on Christmas Day itself.
Chronic Backstabbing Disorder: Simeon Lee, big time. Almost literally, since his younger self attempted to stab his mining partner in the back. He happily boasts how often he has tricked other people, women in particular.
The roles of Alfred and David Lee and their respective wives are combined. David's worship of their mother and Excessive Mourning for her are, though downplayed, now traits of Alfred's; Alfred is also a painter like David was in the book.
The roles of Harry Lee and Stephen Grant get merged. The latter’s only function in the plot was to be a Red Herring and Conchita's love interest, so Conchita is paired up with Harry instead.
Convenience Store Gift Shopping: Poirot has to buy a last-minute gift for Inspector Japp (Jamaican cigars). As it happens, Japp appreciates his present far more than Poirot appreciates his...
Creepy Uncle: Harry Lee is attracted to his niece Pilar and doesn’t hide it too well. She doesn’t seem troubled by his affection, however, and is very fond of him in turn, because she is actually just impersonating his niece.
Easily Forgiven: In the end, Conchita is shown at the choir's concert at Gorston Hall. Apparently, by that point, Alfred and Lydia have no problem with the fact she impersonated their deceased niece and was initially ready to claim at least a part of the Lees' inheritance that she had no right to.
In addition, she is completely Let Off by the Detective (it's unclear whether she used Pilar's passport or her own to enter the country, but her lies could definitely count as obstructing the investigation).
English Rose: Lydia is a polite, industrious, Silk Hiding Steel English housewife, and she possesses a more modest type of beauty compared to Magdalena and Conchita.
Evil Feels Good: Simeon Lee admits to Pilar that he has been a very wicked man and enjoyed every minute of it.
Exact Words: In the prologue, when Stella reveals that she knows Simeon was shot, he says it was because he was attacked by his prospecting partner. He conveniently leaves out the fact that he attacked first.
Hands-On Approach: Harry teaches Pilar/Conchita to play pool by embracing her, putting his hands over hers, and whispering in her ear.
Hollywood Healing: Conchita is severely beaten on the head and gets knocked unconscious but is already able to walk and talk normally on the next day with a bandage on her head, and the day after that, even the bandage is gone.
I'm a Doctor, Not a Placeholder: When Simeon Lee sees that Poirot is not strong or young enough to protect him, Poirot replies:
Poirot: Hercule Poirot is a detective, not a bodyguard, monsieur.
Not Blood Siblings: Just before Poirot's final summation, it's revealed the supposed Pilar is really Pilar's friend Conchita, completely unrelated to the Lees. Harry Lee happily comments that in this case, he isn't her uncle anymore, and they have a Last-Minute Hookup.
Obfuscating Disability: Simeon Lee only stands up whenever he's alone, and acts like an infirm old man in a wheelchair when he has company.
Pretty in Mink: Magdalena and Conchita wear elegant fur coats.
Prolonged Prologue: The episode starts with a prologue set in South Africa in 1896, and it takes nearly ten minutes before we get to the opening title in London, 40 years later.
Ungrateful Bastard: Stella saves Simeon's life, and he robs her and vanishes from her life – after sleeping with her, too.
What seems to be an incestuous attraction between Harry and his niece Pilar turns out to be okay, since the woman reveals she was impersonating Pilar (who is dead).
What seems to be an incestuous attraction between Harry and his niece Pilar turns out to be okay, since the woman reveals she was impersonating Pilar (who is dead).
