In “Dusk,” the protagonist Gortsby believes that an irritated young man is trying to swindle him with an elaborate sob story about being stranded in a foreign city with no money or lodging. When the young man cannot produce evidence (a cake of soap) to verify his predicament, Gortsby superciliously criticizes the man and refuses to give him money. He tells the young man,
The weak point of your story is that you can't produce the soap ... To lose an hotel and a cake of soap on one afternoon suggests willful carelessness
The young man immediately flees. Thinking that he caught the young man in a lie and avoided being conned, Gortsby is proud of himself, until he discovers a cake of soap near where the young man was sitting. It must have fallen out of the young man’s pocket!
Ashamed that he doubted the young man, Gortsby wishes to make amends and runs after him. Gortsby catches up and calls out to the young man:
He turned round sharply with an air of defensive hostility when he found Gortsby hailing him.
Until this point in the story, the young man seems innocent; perhaps he swivels “sharply” and acts defensive and angry because he resents not being believed earlier.
Later in the story, however, Gortsby discovers that the cake of soap was never the young man’s possession in the first place; it is a poor old man’s soap. Therefore, Gortsby’s original impression is correct—the young man is a lying con man. His story about losing the cake of soap is indeed fictional and just a sympathetic ploy to cheat Gortsby out of money.
Thus, in light of the later revelation, the line describing the young man’s reaction is significant because it reveals his guilt. His “sharp” or sudden turn toward Gortsby seems like an instinctive and aggressive gesture. He behaves like a caught criminal. Thinking that Gortsby chased him down in order to nab him or even have him arrested, the young man acts guarded and ready for a fight. Finally, his heated demeanor is consistent with his earlier appearance as a bitter, expletive-spouting man.