One of the reviewers pointed this out and I agreed; Beth's reaction to Kyle's finding champagne in her fridge was decidedly apathetic, as if she expected it. No wonder Kyle thought there was something going on between them.
Was Beth so laid-back that she had no shock? She sure seemed scared when her window was open. Does it take a tripod to fall on her head?
I don't think there's any good explanation for it, other than maybe Beth was in denial about just how dangerous Stephen was at that point. The movie did a good job portraying Beth as this sweet girl who didn't want to hurt Stephen's feelings, but I also think she was naive and didn't fully comprehend the lengths at which Stephen was going to get to her. At that point he had crossed so many lines and yet she continued to be nonchalant about it. Not that any of it was her fault or that she could have prevented it (because Stephen, as the stalker, is the ONLY one to blame), but I think her lax attitude about it all could be explained by denial.
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I don't put movie quotes in my signature.