Symbolism, Imagery, Allegory
Pre-surgery, Charlie views things just a tad differently than everyone else. He's always looking through a window in his childhood, trying to figure out how to get to the other side. It's all about voyeurism for Charlie-boy: watching from a safe distance and taking pleasure in what he can't quite understand. Although Charlie eventually experiences some of the things he observes through the window—like sex—he's never totally able to shake this symbolic separation between himself and others.
And here's the creepy thing. When Charlie starts to regress, he sees the kid at the window more often:
Little Charlie Gordon staring at me through the window—waiting. Please, not that again. (17.296)
It's almost like little Charlie is just biding his time, waiting for an opportunity to reclaim his body and his former life. The window that divides Charlie from his old self starts to seem less like a barrier for his own safety, and more like a menacing reminder of his future.
When the window last appears, Charlie describes himself as watching someone else this time. He watches a lady who "never pulls her shade down" (17.303) as she undresses every night. Not cool, Charlie. Although Charlie feels sexual excitement while watching the lady undress, he also feels a sense of belonging—and it makes him feel sad and lonely when the lady turns off the light and his view from the window vanishes. Charlie is only comfortable experiencing intimacy and other human emotions from a distance, and the window makes that possible.
Watch out for window-washers, Charlie.