One change that I am quite certain that I would like to make is that I would have the film show what Patrick says to Mary. In Dahl's short story, the reader never gets to know exactly what Patrick says that upsets and stuns Mary so much. It's obviously something that ends the marriage. I think a film version should script out that scene. It would help paint Patrick as the bad guy a bit more, and it would increase our sympathies for Mary.
The other change that I would make would be to have Mary's unborn child already born. Mary is already a sympathetic character to begin with because she is pregnant. But all viewers would get to see is Mary worried about herself. Based on the text, she is worried about her unborn baby, but that would be difficult to show on screen.
On the other hand, what about the child? What were the laws about murderers with unborn children? Did they kill then both-mother and child? Or did they wait until the tenth month? What did they do?
Mary Maloney didn’t know. And she certainly wasn’t prepared to take a chance.
If the baby is four months old for example, then the director could have a scene where Mary talks out her thoughts to the baby about covering up Patrick's death. Mothers always talk to their baby, so having Mary "discuss" fears with her baby would not be weird.