Who are examples of flat characters and round characters in "Lamb to the Slaughter"?

There are only two important characters in "Lamb to the Slaughter." They are Mary and her husband Patrick. Mary Maloney is a good example of a round character because she changes quite impressively during the story. She is introduced as a very passive, dependent, devoted housewife who lives for her husband. She fawns on him when he is at home and thinks about him when he is away at work. However, when he tells her,...

There are only two important characters in "Lamb to the Slaughter." They are Mary and her husband Patrick. Mary Maloney is a good example of a round character because she changes quite impressively during the story. She is introduced as a very passive, dependent, devoted housewife who lives for her husband. She fawns on him when he is at home and thinks about him when he is away at work. However, when he tells her, as he obviously does, that he no longer loves her, is bored with their life together, and wants a divorce, she kills him in a sudden fit of blind rage. This change is startling to the reader, who doesn't think she was capable of such direct, independent, decisive action. Then she displays her nerve and cunning when she establishes an alibi and gets the investigating police officers to eat the leg of lamb, the very murder weapon they have been looking for. In the end Mary seems to be enjoying her triumph and her new freedom.

And in the other room, Mary Maloney began to giggle. 

Patrick Maloney, on the other hand, is a flat character, pretty much a stereotypical cop. He is the strong, silent type. He is devoted to his job. He doesn't appear to have much of a life outside of being a policeman, plodding the same beat day after day. The fact that he wants to get out of his marriage to Mary does not prove that he is capable of changing. He handles the breakup with Mary in the same fashion in which he might deal with a woman suspected of shoplifting. He may have feelings, but he is not in touch with them. He has to get drunk in order to say what he has to say to his wife. He does not show her any pity or sympathy at all. He is brutal. But maybe his job has made him that way. If he were leaving Mary because he was having an affair with another woman, that would suggest some change in Patrick's character. But evidently he is not emotionally involved with anyone else, as indicated by his keeping regular hours at home and by his concern about avoiding any hint of misbehavior reaching fellow officers and superiors. He ends his long speech to Mary with these words:

"Of course I'll give you money and see you're looked after. But there needn't really be any fuss. I hope not anyway. It wouldn't be very good for my job."