She smashes the furniture and breaks some glasses to make it look like someone had broken in and calls another person to create an alibi, but in the book she just goes and buys some vegetables to make it look like Patrick died in the meantime. This makes Mary look more like a real criminal rather than someone who was peaceful and happy a few minutes ago, which I believe is what Roald Dahl didn't want people to believe. Dahl wanted people to see this as a comedic mishap upon another, which is what Hitchcock got wrong.
In conclusion, I believe the screen-adapted version of "Lamb to the Slaughter" could have been better in many ways, and it pales in contrast compared to the short story, which was witty and delightful to read. Roald Dahl did a better job conveying the mood and the overall premise of the story, which is why his original story is more eloquent and better-constructed than Alfred Hitchcock's version of "Lamb to the