in the opening paragraph of the story how does the author set the stage?

First of all, let's take a closer look at the opening paragraph of Roald Dahl's short story, "Lamb to the Slaughter."

The room was warm and clean, the curtains drawn, the two table lamps alight– hers and the one by the empty chair opposite. On the sideboard behind her, two tall glasses, soda water, whiskey. Fresh ice cubes in the Thermos bucket.

In this opening paragraph, Dahl sets the scene of cozy, domestic tranquility. The reader discovers later that Mary Maloney, the pregnant wife, is waiting for her husband to come home from his work as a policeman. She appears a good housewife as it is obvious that the home is "warm and clean." The husband and wife seem to be in a comfortable routine; Mary has the drinks waiting. The scene appears very normal and one full of marital comfort.
However, appearances can be deceiving as the reader later finds out when Patrick Maloney gives his wife the news that he is leaving her. Dahl uses the opening paragraph not only to shock the reader as to what occurs later in the story but also to contrast Patrick's apparent discontent with his wife's contentment. The tranquility of the opening scene becomes ironic and filled with emotional tension when Mary kills Patrick in the same peaceful setting with the leg of lamb.