- In the book, when they learn the victim was repeatedly stabbed, Monsieur Bouc immediately labels it a crime of passion and says that the murderer must be a woman. Actually, the crime was ruthlessly planned in advance, but it was a woman who did all the planning and organizing. Well played, Agatha.
- Poirot referring to the Americans as subjects rather than citizens makes a lot more sense when you remember the Belgians have a king, and that Poirot is famous for his Obfuscating Stupidity Funny Foreigner routine.
- Nearly every one of the suspect utters some kind of mistrustful, xenophobic remark in passing — "That's the worst thing about Americans, they're so sentimental" — "I normally don't like them Britishers" — passengers constantly reduce one another to their nationality. But in fact, everyone is working together, regardless of what country they come from, to eliminate a common enemy. This is actually a calculated move to keep anyone from figuring out they had all been part of the same household, and as close as could be despite most of them not actually being related.
- Related to the above, xenophobia works against the investigators as well. Notice how both Monsieur Bouc and Dr. Constantine frequently base their theories of the case around particular national stereotypes (they assume its a crime of passion and rule out the English suspects because they're "cold" and "don't stab people", Constantine suspects the Italian because Italians are hot-headed and do stab people, and so forth).
- Poirot reiterates many times in other books that the reason he never lets a murderer get away with it, however despicable the victim, is that once someone has taken it upon themselves by their own judgment to act as judge, jury, and executioner they become too dangerous to continue in society. In this book, he lets the murderers go, not only because of how much of a monster the victim was and because the wronged parties had already tried to seek justice through legal means and been bitterly let down, but also because they judged and acted as a large group rather than as individuals and hence would not be likely to form a habit of murder in the future.
Fridge / Murder on the Orient Express