There are a number of techniques authors use for creating suspense, some are vocabulary, dialogue, setting, mood and tone. The main mechanism for building suspense in "The Open Window" is through a structural technique. Saki juxtaposes Framton's need for complete mental rest and the horrible story, with its grizzly expectation, Vera is telling to him. The reader knows Framton's need for mental rest and tranquility and feels the building, mentally unsettling, horror of the story of the three beloved men--and spaniel--being lost in a quagmire of the bog. The possibility that is raised that the men and spaniel might come back through the ceiling-to-floor French window continues the suspense once it is begun.
Another structural technique in the characterization of Vera adds further to the suspense. The fact that she asks questions about Framton's knowledge of the area and of her aunt in particular lends a suspenseful chill to the story as it progresses. In addition, her seemingly offhand remark about the open window in October, right after mentioning the "tragedy," adds to the suspense because it makes readers alert to the titular theme (the theme represented by the title): "The Open Window."
"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.