It is clear what makes Poirot such a good detective is his attention to detail. He observes and analyses every person he encounters—not just physical traits, but how a person acts and interacts with others. A good example of this is the relationship between Colonel Arbuthnot and Mary Debenham on the train. Poirot becomes suspicious because of their quick familiarity to each other, were they really strangers?? Poirot immediately observes Mary's cool and calculated demeanor, strict English manners and efficiency too great to be called "joie femme." Later in the novel, these observations make Poirot think this woman might be capable of murder and are essential to his solving the case.
Poirot is a nosy man, a trait that makes him a great detective. The conversations Poirot eavesdrops on, between Arbuthnot and Debenham, help him solve the Orient Express murder later on. Mary's comment, "Not now. Not now. When it's all over. When it's behind us—then—," her anxious behavior when the train stops and her remark to Arbuthnot that she wishes she could enjoy the landscape," all make Poirot suspicious that she is hiding something or about to be engaged in something bad. Poirot's suspicions and Christie's attention to these particular conversations purposely foreshadow the ending of the book.
When Poirot arrives at the hotel, his observations of Ratchett and McQueen in the dining room, prove equally important foreshadowing. Ratchett immediately "captured the detective's attention" and make Poirot convinced he is dangerous, a "wild animal." Poirot's negative feelings and initial reaction toward Ratchett are extremely important because they make the reader and Poirot sympathize with the Armstrong family later in the book. Describing Ratchett as a "wild animal," something less than human, the murder less offensive. the Armstrongs have slayed a beast rather than a human being.