The minor characters in Strangers on a Train are types who function to underline themes and advance the plot: the supportive woman who begins to grow suspicious; the relentless detective; the man who is more interested in not getting involved than he is in justice. The main characters, Guy and Bruno, are the point of the plot, in that Highsmith uses her suspense story as a framework to explore their psyches and their increasingly entangled and emotionally complex relationship.
Highsmith’s writing style is not especially notable. Occasionally she uses images to symbolize aspects of her characters, as when detective Gerard is described waggling his finger at, and so terrifying, a caged bird. Another example is the boil in the middle of Bruno’s forehead, which symbolizes social and psychological realities: Something about him is noticeable to others and found to be unpleasant, something that he himself interprets as “everything I hate boiling up in me.”
Highsmith also reveals character through action. One of the most memorable sequences in the novel involves Bruno’s jubilant behavior on a merry-go-round, singing and shouting, as he stalks Miriam. Conversations, too, are important in providing insights into character, as when Bruno boasts to Guy about a gratuitous theft he has committed. When Guy tells Owen Markman that under certain circumstances anyone can be made to murder, the extent to which he has been influenced by Bruno is underlined; early on Guy had disagreed with Bruno’s assertion that circumstances, and not temperament, are what cause a person to murder. “I’m not that kind of person,” he had told Bruno.
Highsmith’s most common and effective technique of characterization is her use of third-person-omniscient narration to report the thoughts and emotions of her protagonists, creating an effect akin to stream of consciousness, but with an important difference. As she notes in her 1966 book Plotting and Writing Suspense Fiction, “I have quite a bit of introspection in my heroes, and to write all this in the first person makes them sound like nasty schemers, which of course they are, but they seem less so if some all-knowing author is telling what is going on in their heads.” The shift in point of view makes her murderers more sympathetic.