"'I have learned to save myself useless emotion.' She was not even looking at him. Her gaze went past him, out of the window to where the snow lay in heavy masses. 'You are a strong character, Mademoiselle,' said Poirot gently. 'You are, I think, the strongest character among us.'" 

After the passengers wake up to discover that the train is stuck in the snow in Part One, Chapter Five, this exchange occurs between Mary and Poirot in the dining car. Her reserved and to-the-point character is on full display in this moment, especially as she pushes Poirot’s casual conversation aside to contemplate the implications of the snow on their crime. Although the reader is unaware of Mary’s role in crafting Ratchett’s murder at this point in the novel, Poirot’s astute observation about the strength of her character foreshadows her powerful position. 

"Poirot smiled. ‘Ah! But that is only psychological. I ask myself, is it possible for Miss Debenham to have planned this crime? Behind this business, I am convinced, there is a cool, intelligent resourceful brain. Miss Debenham answers to that description.’”

Poirot offers this assessment of Mary in Part Two, Chapter Twelve as he tries to convince M. Bouc and Dr. Constantine that she may be guilty of the crime. They believe she is innocent because she appears so put together and expresses little emotion, but Poirot sees through that façade and picks up on her powerful intelligence. Mary may try to depict herself as distant and uncaring in order to avoid arousing suspicion, but ironically, this performance particularly draws Poirot’s attention. 

"Oh, blame—it is not blame—it is publicity! So far, M. Poirot, I have succeeded in life. I have had well-paid, pleasant posts. I was not going to risk the position I had attained when no good end could have been served." 

When Poirot accuses Mary of lying to him about her real identity as the Armstrong’s governess in Part Three, Chapter Seven, she exclaims that she did so in order to preserve her employability. This argument reflects a level of desperation that seems surprising coming from the unbothered character she portrays throughout the novel, thus suggesting that she had to put substantial effort into disguising her true feelings. Mary still, however, hides the truth about her role in the crime, and this act of self-preservation adds yet another layer to the overall self-centered tone of this quotation.