A motif in the novel is Poirot's insistence that everyone has something to hide. He repeats this statement many times, including at a moment in Chapter 12 when he has several of the suspects gathered together:
["]Every one of you in this room is concealing something from me.” He raised his hand as a faint murmur of protest arose. “Yes, yes, I know what I am saying. It may be something unimportant—trivial—which is supposed to have no bearing on the case, but there it is. Each one of you has something to hide. Come now, am I right?”
Everyone is defensive when he accuses them of hiding things from him. They try to protest, but no one wants to be too loud about it for fear that they will look guilty. But Poirot is not interested in shaming people for their secrets. On the contrary, he believes that uncovering what everyone has to hide will help him clear suspects. After all, he suggests, most of their secrets are "trivial" to the case at hand.
Poirot's belief that everyone has something to hide is foundational to his approach as an investigator. Whereas the police interview suspects only until they have an obvious explanation for what happened, Poirot believes that his job is to treat each piece of testimony like an incomplete puzzle piece. He looks at the puzzle from all angles and seeks to uncover the secrets that allow him to fit the pieces together. Vulnerable as it may feel, Poirot insists that by revealing their secrets to him most of the suspects can in fact help him demonstrate their innocence.
Although Sheppard frequently hears Poirot insist that everyone has something to hide, he does not realize until the end of the novel the extent to which Poirot has been probing for his secrets. As it turns out, Sheppard is the one whose secrets are truly incriminating. Christie took readers entirely by surprise with this twist; until this novel, it was a rule that the narrator of a detective novel could never be the killer. In this sense, the motif of Poirot's suspicion serves as a clue to readers that even the narrator has secrets.
In Chapter 23, Sheppard uses a simile comparing Caroline to a dog. This simile is part of a broader motif of animal comparisons Sheppard uses to describe Caroline:
We went off, leaving Caroline rather like a dog who has been refused a walk, standing on the front door step gazing after us.
The simile does not just serve as a neutral descriptor. Rather, it establishes a power dynamic in Sheppard and his sister's relationship. Caroline "has been refused a walk." Like a dog who is forgotten at home, she is not allowed to go outside except at the invitation of her person. There is an underlying sense that if Sheppard did bring Caroline out of the house, he would have to keep her on a figurative leash to keep her from bothering people or putting herself in harm's way. Sheppard seems to take a cruel delight in his sister's tragic "gaze" and in the empowering act of refusing her an invitation.
When Sheppard first introduces Caroline, he compares her to Rikki-Tikki-Tavi, a mongoose from a Rudyard Kipling story who is too curious and adventurous for his own good. Throughout the novel, he frequently uses these animal comparisons to insult Caroline. The comparisons provide comic relief, but more often than not they also have overtones of misogyny. The mongoose comparison, for instance, is a critique of Caroline's interest in gossip, which is a pursuit often associated with women. The dog simile in Chapter 23 likewise puts Caroline "in her place," so to speak, as a woman. It emphasizes that she belongs inside the house, not out in the world. In also denigrates her and other women by suggesting that women and dogs are alike in this regard.
Christie is generally a lighthearted writer, and she lets Sheppard have some fun at Caroline's expense. She even seems to expect that the reader will laugh at some of these jokes as well. Still, the fact that it is the killer who makes all these jokes suggests that indulging in them regularly is not a very honorable way to conduct oneself.
