Unmarked spoilers are ahead, per Spoilers Off rule.
Poirot comforting Linda after her failed suicide attempt by assuring her that she's not the wicked person she thought herself, only a normal young woman who was trying to deal with completely understandable anger.
In the 1982 film, when Poirot solves the crime and returns Sir Horace Blatt's stolen diamond, the normally-stodgy Sir Horace is genuinely grateful:
Sir Horace Blatt: You're a wonder, Poirot!
In the original novel, Linda indicates (in her typically understated way) that she will enjoy having Rosamund Darnley as her new stepmother. Similarly, the 1982 film has Kenneth and Linda present during Daphne Castle's final scene, with the implication that they may well become a family.
In the novel, Captain Marshall's proposal to Rosamund (or her's to him depending on how you look at it) can count, too. As mentioned under Values Dissonance, it can rub audiences today the wrong way with how Kenneth makes it sound like he's forcing her to choose between her successful career and a life with him in the country but (a) Rosamund's response is that she's always wanted to live in the country with him and earlier conversations with Poirot imply this is true and she may even have been feeling Lonely at the Top and (b) a bit more up for interpretation on Kenneth's side, this seems to be him, in his own emotionally clumsy way, asserting himself and saying he's not going to deal with the nightmare of another relationship where his wife gets to do whatever she wants without a care for his feelings.
