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.