Christie makes explicit mention of Mayherne and Romaine’s unconscious habits. She establishes Mayherne’s idiosyncrasies twice in the first page: “Mr. Mayherne adjusted his pince-nez and cleared his throat with a little dry-as-dust cough that was wholly typical of him” (1), and, later, “He coughed again, took off his pince-nez, polished them carefully, and replaced them on his nose” (2). Later, when Mayherne goes to visit Mrs. Mogson, he twice notes “her hands clenching and unclenching themselves nervously” (20).

In the case of Mayherne’s glasses, there is a symbolic significance to the action, in that he tends to polish the pince-nez while “thinking deeply” (6). The habit therefore reflects his efforts to “see” the facts of a case clearly, and it is indeed while cleaning his glasses that he realizes Romaine’s deception. Following Leonard’s acquittal, Mayherne finds himself “polishing his pince-nez vigorously” (26), and reflects that habits are “curious" in the sense that “[p]eople themselves never knew they had them” (26). He then recalls Romaine on the stand:

If he closed his eyes he could see her now, tall and vehement, her exquisite body bent forward a little, her right hand clenching and unclenching itself unconsciously all the time. Curious things, habits.