I cannot BELIEVE they changed the ending of this story! If you don't want to know how the book ends, read no further.
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so, in the book (a) there is no "lab" in the cellar, it's just where the husband develops photographs, and there is no problem with her going down there. In fact, that's where he plans on killing her, by getting her to go down there with him to help him with his work. (b) (a minor point) there is no "dark and stormy night". (c) (a HUGE thing) the wife figures out what is going to happen at the last moment, and there is the meal, and the coffee. But she had (in a very clever way and with her husband sitting right there) managed to get a message to her ex-fiancé, and was hoping he'd get there in time to help. She just needed to stall her husband for as long as possible, so when he started pushing her to go down to the cellar, she told him that she had a confession to make. She told him that she had been married twice before, and (in much more detail), that she had actually poisoned her first two husbands. She had been a nurse in the war, and knew about drugs. At this point the husband is starting to get very nervous. A wife murderer trapped by a husband murderer! She told her husband that the drug was a paralytic, and that he would be unable to move from his chair. He was so scared that he believed her, and actually couldn't move. She kept talking to him, telling him what the drug was doing to him, until her fiancé arrived with the police, and found her sitting there with her husband, who had died.
Maybe not the most plausible scenario in the world, but REALLY cool, and suspensefully written. Instead, the movie had to make the woman this poor, helpless creature that had to be saved by the men in a big dramatic fight at the end. On a dark and stormy night, of course. Grrrrrr.