"What did it matter after all? And wasn't he utterly sick of himself?"

Fifty years before the age of the internet, Highsmith explored the dark designs that can emerge from encountering a stranger and understanding only a little information about them. This quotation also suggests that Guy's sickness was already present before he met Bruno, indicating that Guy had a propensity toward crime prior to the circumstances that pressure him into the murder.

“How boring it was really, Guy thought, crime. How motiveless often. A certain type turned to crime. And who would know from Bruno’s hands, or his room, or his ugly wistful face that he had stolen?” 

Early in the novel, Highsmith introduces a theme consistent with several of her other novels, which is to question the notion that "other" people commit serious crimes. Highsmith shows that it is the fragility of civility that accounts for the tenacity with which society holds this belief. Her principle character bears a name that stands in for society at large, subverting the typical modernist focus on the primacy of individual perspective.

"The stranger on the train who would listen, commiserate, and forget."

A key quotation in a novel about strangers, Guy's assumption echoes another rule of polite social relations: disinterest. In Chapter 1, as in the final chapter with Owen Markham, Guy solicits the therapeutic ear of a stranger. In both contexts, these recall the analyst's consulting room. The dangers of listening are revealed when the train carriage and hotel room become scenes for the dramatization of Guy's soul.