In the 1974 Murder on the Orient Express, every time Poirot finishes questioning a suspect, Bianchi (I think) says "It was (her/him)! (He/she) did it!" He's right, of course, every time.
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 StupidityFunny 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.