Most of the characters in Murder on the Orient Express affect the conflict because they keep secrets about their true identity and play important parts in Ratchett’s murder. There are two notable exceptions: Monsieur Bouc, director of Compagnie Wagon Lits, is a former colleague of Hercule Poirot’s and brings him into the case immediately. The other is Dr. Constantine, the coroner who helps in the investigation.
The rest all tell a series of lies in order to cover their guilt. Slowly Poirot catches them by realizing the smallest inconsistencies in their stories. Everything centers around the kidnapping and murder of young Daisy Armstrong. The major conflict involves the question of who killed Ratchett, the man who was responsible for her death and got away with it.
Each character confuses the issue in order to fool Poirot. For example, Mrs. Hubbard is really Daisy’s grandmother; she makes up a story about a man in her cabin. Mary Debenham had been Daisy’s governess. Princess Dragomiroff spins numerous false stories about the other passengers. Hildegarde Schmidt is not really a lady’s maid; she had been the Armstong family’s cook.
In essence, the characters in the story add to the conflict by muddying the waters and confusing the clues as much as they possibly can. It takes all of Poirot’s ingenuity and patience to untangle what really happened on board the train that night.