That whole "things are not always what they seem" thing goes double for Murder on the Orient Express. Since we know the murder was committed by someone on the train car, we know that someone must be lying. But, as we eventually find out, almost everyone on the train was lying. It's as though everything that happens on the train is a play being acted out by the conspirators. Of course, it's not really a play, as it involves a real-life murder. Fortunately, our detective, Hercule Poirot, judges appearances remarkably well. His keen eye for observation reveals the cracks in the surface of the conspirators' facade.
The passengers underestimate Hercule Poirot because of his appearance.
The novel suggests that people should never judge based on appearances.