Dahl’s writing style in “Lamb to the Slaughter” is primarily simple and direct. He tells the story via a third-person limited narrator who stays close to Mary’s experience, capturing Mary’s thoughts, as well as the external events of the story , in short sentences and everyday language.
The moments when Dahl’s style changes are significant and usually note a shift in Mary’s internal experience. Take the following passage, for example, which comes as Mary is returning from the grocery store and trying to convince herself that she is unaware that her dead husband (whom she killed) is waiting for her at home:
And now, she told herself as she hurried back, all she was doing now, she was returning home to her husband and he was waiting for his supper; and she must cook it good, and make it as tasty as possible because the poor man was tired; and if, when she entered the house, she happened to find anything unusual, or tragic, or terrible, then naturally it would be a shock and she'd become frantic with grief and horror.
Dahl’s normal style shifts in this passage, as seen in the way that he extends the sentence, using semicolons to capture Mary’s agitated state. The long sentence and semicolons communicate that Mary is anxiously jumping from thought to thought, a real departure from the calm and collected state of mind she was in earlier in the story. Though Mary is trying to convince herself that she is calm and collected (and didn't do anything wrong), the style of the sentence suggests that she is quite anxious about what she did (and what the consequences could be).