Hercule Poirot, a recurring Christie character, has become one of the most famous fictional detectives. Poirot is a retired Belgian police officer turned private detective. As a private detective he tours Europe and the Mid-East solving murder mysteries. Because he is a private detective and has no apparent family, Hercule Poirot has a great deal of freedom. He is independently wealthy and the decisions he makes are not subject to law or otherwise. As exemplified in Murder on The Orient Express, Poirot does not always follow the law—he lets the real murderers go. This novel is one of two Christie books where the murder is let off. While Poirot does not always obey the law, he always abides his conscience and his sense moral law. "Moral Law" is somewhat like religious law or the law of God, it is a general sense of right and wrong that supersedes any man-made written laws. In the case of the Armstrong family, Poirot put moral law first. The private detective is an arbiter of morals; he has the power and the brains to fight evil.
Poirot is moral and intellectual superhero. He is quite clearly smarter than any of the other passengers, especially M. Bouc and Dr. Constantine. In the beginning of Section three, Christie includes a humorous comparison of the thoughts of the three men. While Poirot sits motionless thinking and concentrating on the case, M. Bouc's thoughts wander to the repair of the train and Dr. Constantine's waver into pornography. Poirot's greatest task as a detective is to be the smartest person around; he must intellectually defeat the murderer. The Armstrongs purposefully attempt to confuse and fool Poirot. They set an elaborate set of clues and misleading evidence to veer him from the truth, but Poirot still wins. From the time he sits down and "thinks" with Dr. Constantine and M. Bouc, Poirot knows the solution of the case—it is merely a matter of confirming his suspicions.
Poirot is a very likable character, despite his moral and intellectual greatness. He is over concerned with appearance, distracted by his moustache and has a liking for strong-willed British women (a.k.a. Ms. Debenham). He is rather short, slightly snobby and probably lonely at times. It is good Christie gives him cases so often. Hercule Poirot, through Christie's novels, is said to have aged to 105.
Mrs. Hubbard, the character played on board the Orient Express by Linda Arden, famous actress and grandmother to Daisy Armstrong, is a comedy of the "American woman." Mrs. Hubbard is the only admittedly American woman on the train. Linda Arden heightens the character's Americanisms, Mrs. Hubbard is loud, need constant attention and espouses Western ideals. The first time Poirot encounters Mrs. Hubbard she is talking about the US, "you can't just apply American methods in this country. It's natural here for folks to be indolent. They haven't got the hustle in them...We've got to apply our Western ideals and teach the East to recognize them." Mrs. Hubbard uses less slang than Hardman, but still throws in an occasional "folks."
The character of Mrs. Hubbard is instrumental in the planning and carrying out of the murder. Mrs. Hubbard's cabin is right next to Ratchett and shares a communicating door with him. The night of the murder Mrs. Hubbard tells Poirot that Ratchett is a monster and that she is scared of him, she plants the idea that Ratchett is a bad person is Poirot's mind. The reader knows that Poirot already suspects Ratchett of evildoings, but Mrs. Hubbard does not. Mrs. Hubbard's call to the conductor in the early morning hours is also important to the case. By saying there was a man in her compartment, Mrs. Hubbard removed herself from suspicion since she was a victim of the attack. Mrs. Hubbard's hysterical behavior makes it easy to dismiss her as a suspect. Only an extremely talented actress could ever make up such fantastical speeches as Mrs. Hubbard. Mrs. Hubbard's downfall comes from one, simply mistake—the story about the lock on the communicating door. If Linda Arden hadn't said the bolt was below her bag, the case might have turned out differently. This piece of evidence, this obvious lie, called her into suspicion and confirmed her participation in the crime.
Mary Debenham, the tormented and unhappy spinster stock character, is actually the most attractive and interesting female on the Orient Express. Poirot describes Mary as "cool and efficient," a formal and somewhat uncaring English lady; however, Mary is also revealed as an extremely passionate woman. Poirot is attracted to Mary's defiance and quick mind, when she enters the dining car for questioning, he describes her, saying, "She wore not hat. Her head was thrown back...The sweep of her hair back from her face, the curve of her nostril suggested...a ship plunging gallantly into a rough sea...she was beautiful."
Mary is an odd combination of a cool, plain governess and femme fatale. When executing his questioning, Poirot is hardest on Mary—he gives her no breaks and no apologies and his behavior is much different with her than with the other passengers because he know just how strong she is and how difficult she will be to break. Mary was the principle planner of the murder, if anyone she is Poirot's closest match. Poirot does not defeat her until his second round of questioning. This second round is like Mary's last stand, it is the same described above where she is "beautiful." The pressure seems to finally crack her iron demeanor and it is up to Colonel Arbuthnot to defend her.
The fact that Mary is in love with Colonel Arbuthnot does make her elevate her above the typical spinster character. The spinster often isn't a very lovable character, but Mary clearly can be loved and wants to be loved. The presence of a male partner makes the female seem gentler, more digestible. Because Mary breaks into tears and Colonel Arbuthnot must rescue her, the reader feels more sympathy for Mary—she is not too independent or masculine.