A frame story, the ironic "The Open Window" is dependent upon its main character, Vera, for its development. The other characters are merely present as the audience for Vera's ingenious fable. In fact, they resemble stock characters; that is characters of a type quickly recognized by the reader that require no development by the writer.
Vera, Saki's mischievious protagonist, is indeed a dynamic, or round, character who exhibits several traits. For one thing, she is a perspicacious young lady who is able to identify people's vulnerabilities and seize upon details surrounding her in order to weave a convincing tale that will deceive her audience. She, also, can quickly adapt her fabrications to the next person or situation as her spontaneous response to Mrs. Stappleton's inquiry about Nuttel's bizarre reaction when she has announced the return of her family. Certainly, Vera enjoys exploiting the weaknesses in others with her subtle and clever practical jokes.
Framton Nuttel is the nervous, insecure type. He worries about Mrs. Sappleton's reception of him when he is introduced, and he is uncomfortable throughout the telling of Vera's tale.
Mrs. Stappleton is the blase gentlewoman, concerned only with herself and those for whom she cares. For, she takes no interest in Framton Nuttel and shallowly "rattled on cheerfully about the shooting and the scarcity of birds...." When Nuttel mentions his condition as explanation for his visit, she responds in a voice that barely stifles a yawn. After Nuttel's abrupt flight, she merely remarks upon his behavior and displays no concern for his feelings, expressing a slight curiosity for his actions which Vera's cleverly allays.