The Open Window has two central characters, Vera, Mrs. Sappleton's niece, and Framton Nuttel, the visiting stranger who is in the country to rest his nervous condition on the instruction of his doctor. His sister has written him letters of introduction into homes of families that she knows, Nuttel does not know the people he is visiting.
The author does not tell the reader that Vera, the young niece of Mrs. Sappleton who keeps Nuttel company before her aunt comes down to greet him, is a very talented storyteller and that she has taken this opportunity, since Mr. Nuttel is a stranger in town, to tell him a tall tale. We don't know, as readers, that Vera is just spinning a tall tale to actually entertain herself at the expense of poor Mr. Nuttel.
The reader has no way of knowing that Vera is telling a made-up story because we don't know that Mrs. Sappleton's husband and her brothers are alive and just really out hunting and not missing in the swamp for three years. So this lack of knowledge helps to set the reader up for a big surprise and you think that the story is actually a little scary because the suspense is built around the open window which stands, according to Vera, open, waiting for the return of the three long missing men, actually their ghosts are expected.
Nuttel is the character with the greatest detail so that the reader can understand his response and reaction to Vera's story and why he runs out of the house when he sees the hunting party approaching the open window.