I was also dispappointed with Anthony LaPaglia's portrayal as Bob, but I'm not sure if it was LaPaglia's fault or the script's.
I expected to see so much more from Bob's character. In the book, he's not just a cold killer. When he confesses the murders to Darcy, he is sad, and at the same time, you can tell that he really loves her and cares for her. He's a dangerous murderer, who, however, loved his wife so much that he was willing to stop the killings for her - although that was only for a certain period and not forever, but still... I didn't get that from the movie. In the movie, he was talking about his fantasies without any expression, really, and seemed like he didn't even care. I couldn't get what were his emotions and I never even felt that he loved her. That was a huge drawback of the movie, for me.