When Saki was plotting "The Open Window" he must have given considerable thought to creating the character who would tell the spooky story to Framton Nuttel. He chose to give the role to Vera, a fifteen-year-old girl. He may have decided against using a boy because a boy would have been more likely to go off bird shooting with the three men. A girl is more convincing because girls generally appear to be better behaved, although they may harbor all sorts of mischievous thoughts. Vera had to be young enough to play such a trick on a visitor and to take a risk of getting found out after the fact. But she had to be old enough to be entirely credible. Fifteen seems like exactly the right age. She is described as very "self-possessed." Saki uses the term "self-possessed" twice. We picture her as calm, cool, relaxed, quite sophisticated for her age. This is for the sake of contrast with Framton Nuttel, who is just the opposite of calm, cool, and relaxed. Vera's description as self-possessed will also serve as a contrast with the way she behaves when she sees the three men approaching the open window.
Framton is seated with his back toward the open window when Mrs. Sappleton cries, "Here they are at last!" Instead of looking at the window, Framton turns and looks at Vera. The girl is anything but self-possessed.
The child was staring out through the open window with dazed horror in her eyes.
Vera is a good actress as well as a good story-teller. She must have been planning to fake a look of "dazed horror" from the start. It is the look of horror on her innocent young face that frightens Framton more than anything else. All he needs is a glimpse of three men approaching with guns to make him flee in blind panic.