How does the author create suspense in "The Open Window"?

The author
creates suspense by using a narrative style that is unique and characters who are believable.
The narrative structure is that offrame story. Usually, in a frame story we have one narrator
and it is impossible to check to see if that narrator is truthful. Here, Vera seems to be
truthful when she tells Nuttel the story of Mr.Sappleton and the...