Hercule Poirot is the detective, functioning to guide the reader through the clues. Although the book is told from the third person point of view, Poirot is the character who the reader gains the most insights into. This is frequently made possible through his dialogue with two other characters, Dr. Constantine and M. Bouc, who provide an outlet for Poirot to voice his thoughts and theories. By providing their own theories, these characters also give Poirot the chance to correct errant assumptions and keep readers on track as they try to solve the mystery themselves.

Through Poirot’s character, the theme of The Psychology of Investigation is crystallized. Poirot comments on the nature of a locked-room mystery more than once, flagging the fact that they can’t conduct background checks but must rely on psychological assessments.

M. Bouc, the director of the train company, and Dr. Constantine, the doctor who examines Mr. Ratchett (Cassetti)’s body, accompany Poirot as he solves the case. The men frequently suggest their own theories regarding the murder. Bouc and Dr. Constantine act as a sounding board, creating opportunities for dialogue, so that Poirot can verbalize and explain his theories.