Suspense is heightened by the introduction in Vera's story of the supernatural theme. Vera introduces the idea of the supernatural when she says of the three hunters that "their bodies were never recovered" and when she suggests that Mrs. Sappleton expects to see the three hunters return through the open "window just as they used to do" any day. Vera heightens both the theme and the suspense further when she describes just how they went out and, thus, just how they will return and the "creepy feeling" she has that they "will all walk in" through the open window:
her husband with his white waterproof coat over his arm, and Ronnie, her youngest brother, singing 'Bertie, why do you bound?'
Two motifs that recur are the open window and Framton Nuttel's "rest cure" for his nervous complaint. The titular (i.e., in the title) open window motif heightens suspense partly because the window is the dominant thing in the room, extending as it does from ceiling to floor and being wide open as it is on an October day, and partly because of the way Vera looks at it and the subdued, awed tone in which she speaks of it:
"You may wonder why we keep that window wide open on an October afternoon," said the niece, indicating a large French window that opened on to a lawn.
The other, and central, motif the recurs is Framton's rest cure for his nerves. The second paragraph introduces the reader to his rest cure and to the dubiousness he feels about visiting strangers to help his cure. Then we learn from his sister that his rest cure focuses on his nerves, which "will be worse than ever" if his rest cure is not conducted correctly. It comes up again after Framton listens to Mrs. Sappleton "rattle on cheerily" about birds and shooting in a "purely horrible" manner while ever on the look-out for returning hunters, hunters Framton knows as dead corpses. This heightens suspense because we worry with Framton about what will become of him if the ghosts do come through the open French window.
"The doctors agree in ordering me complete rest, an absence of mental excitement, and avoidance of anything in the nature of violent physical exercise," announced Framton,....