The Story That Couldn't Make Up its Mind
I've never read the book, and I sincerely hope it was more fleshed-out than this movie. The movie couldn't decide what story line to pick -- the psycho-obsessed woman after the married man, the murder mystery, the rocky marriage, or the fact that Jill was getting a taste of her own medicine.
So Jill doesn't go to the police with the package she found on her desk filled with glass and pictures? She doesn't even think this is important enough to notify Derek that this woman is dangerous. Or maybe she did and he was just to whipped to see the truth. Either way, she should have taken it to the police.
With that said -- the whole second story-line with the boss and abused wife was just way out of left field unless something more important went way over my head. The only thing I can think of is that it threw fuel on the fire to an already rocky relationship with Jill and Derek.
Also, the the other sub-plot of the mysterious person who gave Jill her butterfly necklace. The audience knows it was her step-daughter, but they made such a big deal about her keeping it a secret we were tricked into thinking Derek would make a big deal about it. Especially when she says "a friend" gave it to her, they pan over Derek's concerned face, and then they drop it and never return to it at all in the movie.
And as for the ending, it's as if everybody forgot Nicole was PSYCHOTIC! What was the point in even making her psycho? It didn't do anything for the story. The entire thing could have occurred without the scenes with the glass and weird pillow love fest thing and nothing would have been lost in translation.
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I don't put movie quotes in my signature. I'm not a hipster like you.