In the enotes biography on Saki, this is written,
Saki came to the short story as a satirist and never averted his eye from the darker side of human nature, a place where not only social ineptness, pomposity, and foolishness are rooted but criminality as well.
This penchant of Saki's for satire seems to be why his stories contain characters who are static, for if they were to possess dualities or varying dimensions, then the point of ridicule could not be as direct and, therefore effective. In his short story, "The Open Window," for instance, Framton Nuttel is small, neurotic man who is the target of Saki's satire by wit of Mrs. Stappleton's niece. His nervous condition and fears allow Vera to easily blur the lines of reality in her fabricated story and, thus, terrify the weak man.
Likewise, in Saki's story "Dusk," the target of satire, the cynical Gortsby, who feels himself superior to the others who frequent the park after twilight in Hyde Park has his superciliousness which at first seems accurate, is later mocked by the ironic turn of events at the end of the story. As Gortsby's outstanding characteristic is his sardonic nature, the effect of its attack is more powerful because of its singleness. Certainly, with the attack of Saki's satire being upon one salient characteristic in the personages of his narratives, the irony of his endings cannot be missed and, thus, the impact of his stories is most effective. Again, enotes comments,
Characteristically, he manipulates a single aspect of individual or group behavior until its irrationality, absurdity, or shoddiness is exposed.