The insanity that comes on the woman in this excellent classic is definitely something that grows gradually. It starts of with the confinement of the narrator to her room for "rest," which, according to John, the narrator's husband, is all that she needs. This is typical of the views surrounding depression of the time, and the narrator is confined to her bed and not allowed to see her baby. However, the first section ends with the narrator expressing her dislike of the wallpaper:
It is dull enough to confuse the eye in following, pronounced enough to constantly irritate and provoke study, and when you follow the lame uncertain curves for a little distance they suddenly commit suicide--plunge off at outrageous angles, destroy themselves in unheard-of contradictions.
Note how this impression of the wallpaper develops gradually. At the end of the second section, the narrator discerns a "strange, provoking, formless sort of figure" that exists in the second layer of design. Gradually, as the narrator's obsession with the wallpaper intensifies, she says:
There are things in that paper that nobody knows about but me, or ever will.
She has discerned that the figure in the wallpaper that she sees is "like a woman stooping down and creeping about behind the pattern." Then the narrator becomes paranoid, thinking that John and Jennie want to discern the secret of the yellow wallpaper before she does. However, the wallpaper now seems to be a cause for health in the narrator, rather than something that overtly makes her worse. However, the narrator detects the "smell of the wallpaper" permeating the entire house, which is a "yellow smell." The narrator finally can discern that the exterior pattern on the wallpaper is moving because the woman trapped behind it shakes it:
And she is all the time trying to climb through. But nobody could climb through that pattern--it strangles so; I think that is why it has so many heads.
Finally, the narrator decides to free the woman trapped behind the wallpaper. One day, she locks her room and throws the key out of the window. Although she pulls a lot of the paper off the wall, and the story ends with the disturbing image of the narrator creeping around the edge of the room by the wallpaper, having to step over the fainted body of her husband at each circuit.
Note how the increasing identification with the wallpaper creates a kind of Gothic "double" whereby the narrator projects her own situation into the life of the woman who is trapped behind the wallpaper.