How does Dahl describe Mary’s characteristics? What textual evidence does the author use to describe Mary and her house in order to achieve this effect?

Dahl first describes Mary as a gentle angel of the home by describing such externals as her physical appearance, activities, and demeanor. She bends over her sewing, a domestic task, looking "curiously peaceful." She is sixth months pregnant, and her calm eyes seem larger and darker than ever. Everything about her exudes a gentle, madonna like quality.
We are shown too how Mary waits on her husband when he gets home, hanging his coat and making his drink.
Dahl's narrator also gets inside Mary's head to record some of her thoughts. For instance, she is looking forward happily to her husband returning from work. We learn that the time of day when he comes home is "wonderful" for her.
However, Dahl shows that while Mary's calm, cool exterior never changes, her emotions do, turning from love and appreciation for her husband to intense anger. After she learns he is leaving her, she whacks him on the back of the head and kills him just as calmly as just made him a drink—and then just as calmly cooks the murder weapon for the police to eat.
At the end of the story, when we are left with Mary laughing over having deceived the police, we understand that she may be externally calm but she is not all innocent.
Although this is a seemingly simple story, Dahl paints a portrait of woman whose actions show she is more complicated than her innocent exterior might suggest.

In order to answer your question, I will first define characterization and what you should look for in order to assess a character. Direct characterization occurs when an author tells the reader what a character is like, while indirect characterization occurs when an author demonstrates what a character is like. A reader should focus on a character’s words, actions, appearance, thoughts, and relationship with other characters to assess his or her character.
In Roald Dahl’s “Lamb to the Slaughter,” Mary Maloney is a dutiful, pregnant housewife to her police officer husband. Throughout the story, Dahl relies on indirect characterization to reveal her traits. Chiefly, Dahl’s use of the third-person limited omniscient point of view gives the reader direct access to Mary’s thoughts and feelings as she awaits her husband’s return from work. For example, one could infer from her anxious thoughts at the beginning of the story that Mary doesn’t like being alone.
In addition, Dahl describes the details of the Maloney home in a way that makes everything seem clean and organized. The specific way in which Mary arranges her home, one might argue, demonstrates her meticulous attention to detail—a trait that serves her well after her husband’s accidental death.
I hope these suggestions allow you to fully understand how Dahl lets the reader infer what kind of person Mary Maloney is despite the expectations that are placed upon her by others.