The most prominent theme in this incident is guilt. Dunstan Ramsay feels responsible for Mrs. Dempster's condition. He remembers how, as a child, he listened "guilt-ridden" to his mother's tales about the first six months of Paul Dempster's life. Ramsay, weighed down by his Presbyterian conscience, carries this guilt throughout his life, providing support to Mrs. Dempster until her passing.

In contrast, Mrs. Dempster's son Paul runs away before he turns ten because his father blames him for his mother's insanity, and he suffers from the cruel jokes of those who mock her condition. Staunton's reaction offers a third perspective. When the young Ramsay confronts him about his actions, Staunton refuses to acknowledge any responsibility for Mrs. Dempster's situation and quickly dismisses the incident. This allows him to join other youths in Deptford in calling Mrs. Dempster a "hoor" without any remorse or guilt, as he remains unaware of his part in her story. As a seventy-year-old business magnate, he is genuinely shocked to learn about Mrs. Dempster's history.

For Dunstan Ramsay, Mrs. Dempster evolves into more than just a responsibility; she becomes his personal saint. As a young boy, Ramsay is drawn to the romantic allure of religion, which he sees reflected in the lives of saints. He entertains young Paul Dempster, who is only four, with card tricks and a delightful book titled A Child's Book of Saints by William Canton. To Ramsay's imaginative mind, the stories of saints offer the magical quality of Arabian Nights that is absent in his family's rigid Presbyterian faith. Hagiography becomes a lifelong passion for Ramsay, and as an adult, he authors popular books about saints and contributes scholarly articles on their lives to the Jesuit Bollandist Society. Mrs. Dempster becomes his "fool-saint," a term introduced to the adult Ramsay by Father Regan, the Roman Catholic priest of Deptford. By then, Ramsay is convinced of Mrs. Dempster's sainthood, attributing three miracles to her, including his own recovery from a five-month coma after being wounded and burned in World War I. He consults Father Regan, who classifies Mrs. Dempster as a fool-saint: "someone who appears full of holiness and loves everyone, doing every good deed possible, but because they're a fool, it all amounts to nothing—or worse than nothing, as it is virtue marred by madness, and you can't predict where it will lead."

The idea of Mrs. Dempster as a saint becomes an obsession for Ramsay, overtaking the notion of her merely being a victim of his impulsive action. This is clarified toward the novel's end when Ramsay has a final conversation with Padre Ignacio Blazon, a Jesuit scholar with the Bollandist Society, who is now over a century old. Padre Blazon's view on Mrs. Dempster is more positive than Father Regan's. After inquiring why Ramsay has never written about his fool-saint, Padre Blazon explains that Mary Dempster qualifies for sainthood on two grounds: she has served as a saint in Ramsay's personal mythology, and she has lived her life in a saintly way. Regarding the miracles, Padre Blazon tells Ramsay, "you believe in them, and your belief has filled your life with beauty and goodness; excessive scientific analysis won't help you. What seems far more important to me is that her life was lived heroically; she faced a harsh fate, did her best, and continued until her madness became too overwhelming. Heroism in God's cause is the mark of the saint, Ramsay, not conjuring tricks."

In Dunstan Ramsay's life, Mrs. Dempster plays multiple roles, not so much through her actions but through how he perceives her. The theme of role-playing has two aspects: the roles we assign to others, like those Ramsay attributes to Mary Dempster in his personal mythology, and the roles that are assigned to us. One of the most surprising roles Ramsay finds himself in is that of a hero. During World War I, he inadvertently disables a German machine-gun position amid the chaos of battle. In the post-war period, society looks for heroes, and he becomes one. When he receives the Victoria Cross from the king of England, Ramsay sees both himself and the king as symbolic figures, "unreal yet very necessary; we have obligations above what is merely personal, and to let personal feelings obscure the obligations would be failing in one's duty." Public figures are given roles as though they are actors in a play, "and it is only right to consider them as players, without trying to discredit them with knowledge of their off-stage life."

In professional settings, people are also assigned specific roles. Although Ramsay does not delve deeply into his teaching career, he does explain why he had to resign from his interim headmaster position at Colborne during World War II. He mentions that he was perceived as an eccentric schoolmaster—known for his unusual attire, distinctive habits ("that disgusting trick of blowing your nose and looking into your handkerchief as if you had expected to prophesy something from the mess"), and expertise in an uncommon subject, saints.

In Fifth Business, characters not only receive roles but also choose how they wish to be seen by others, often signified by changing their names. Three significant name changes occur throughout the narrative. Firstly, Ramsay alters his name from Dunstable to Dunstan following a heroic act on the battlefield during World War I. After enduring a five-month coma, he awakens to find himself physically altered, having lost a leg and sustaining severe burns on his chest. His last memory before slipping into unconsciousness was seeing Mrs. Dempster's face on a statue of the Virgin and Child. Upon regaining consciousness, he feels he was in a special protected realm, watched over by a Madonna resembling Mrs. Dempster. He attributes his recovery not to medical intervention but to himself, "the little Madonna," or "some agencies other than good nursing and medical observation." He briefly engages in a relationship with his English nurse, Diana Marfleet, who proposes he change his name from Dunstable to Dunstan: "St. Dunstan was a marvellous person and very much like you — mad about learning, terribly stiff and stern and scowly, and an absolute wizard at withstanding temptation. Do you know that the Devil once came to tempt him in the form of a fascinating woman, and he caught her nose in his goldsmith's tongs and gave it a terrible twist?" Ramsay is attracted to "the idea of a new name"; it suggests "new freedom and a new personality," akin to a miraculous transformation.

The other two name changes in Fifth Business symbolize the roles the characters choose for themselves. Percy Boyd Staunton adopts the name Boy Staunton during World War I, "and it suited him admirably. Just as Childe Rowland and Childe Harold were so called because they epitomized romance and gentle birth, he was Boy Staunton because he summed up in himself so much of the glory of youth in the postwar period." For Staunton, the name represents his connection to an era and a character type depicted in F. Scott Fitzgerald's stories. As he grows older, the name becomes less fitting, but the notion of someone with a naive and undeveloped understanding remains appropriate for his character.

The third character to change his name is Paul Dempster, who becomes Magnus Eisengrim. Unlike Ramsay and Staunton, whose name changes are subtle adjustments that allow them to leave their mark on the names given by their parents, Dempster completely severs ties with his family by running away. He transforms into a magician, an artist who travels the world. His new name bears no connection to his original one; the identity he presents to the world is purposefully crafted to conceal his true self.

When Ramsay meets Magnus Eisengrim at a magic show in Guadalupe shortly after World War II, it takes him some time to confirm that Eisengrim is actually Paul Dempster. Ramsay is intrigued by how Dempster has developed this "new self," but he never uncovers the full story. Eisengrim is portrayed as a mysterious character in the novel, a completely fabricated identity with ambiguous intentions. Ramsay is tasked with writing Eisengrim's autobiography, a fictional piece that matches the magician's stage persona. In the novel's closing scene, Eisengrim tells Boy Staunton that his name "comes from one of the great northern beast fables, and it means Wolf." There is always something sinister about Eisengrim, the man with the invented name. Having escaped his past and created a new identity, he remains the novel's most enigmatic character.

In addition to being seen as both a victim and a saint, Mrs. Dempster plays another crucial role in Dunstan Ramsay's personal mythology: that of a lover. As a child, some of Ramsay's guilt about the accident stems from his religious upbringing. He learned to be "mistrustful of whatever seemed pleasurable in life," especially concerning sexual matters. Due to the snowball incident, the young Dunstable Ramsay (as he was then called) feels "directly responsible for a grossly sexual act — the birth of a child." After the child is born, young Ramsay frequently visits the Dempster household, assisting Mrs. Dempster with caring for the baby, Paul, and helping with various chores. Looking back as a seventy-year-old memoirist, Ramsay confesses that he was in love with Mrs. Dempster. It wasn't like some boys who admire older women from a distance, indulging in fantasies where the woman is idealized. Instead, his feelings were intense and immediate. He perceives her as his unintentional creation, feeling that he "must hate her or love her." This sense of ownership persists into his adulthood when Mrs. Dempster becomes his responsibility. Ramsay finances her care and visits her weekly. At this point, he sees Mrs. Dempster as an essential part of himself: "a part of my own soul that was condemned to live in hell." There is also a sense of possessiveness, similar to that of a jealous lover. He reveals that he did not ask the affluent Boy Staunton for help with Mrs. Dempster's care because he wanted her to remain his: "I was determined that if I could not take care of Mrs. Dempster, nobody else should do it."