In Roald Dahl's "Lamb to the Slaughter," the suspense is introduced after Mary kills her husband. The decision to hit him with the frozen leg of lamb comes upon her quickly without any warning or premeditation. However, once the deed is done, Mary takes steps to act normal: she adopts a demeanor of heartbreak and innocence; then the suspense begins to slowly build.
Mary rehearses what she will say to the grocer, the man who will be something of an alibi: for after Patrick is dead, Mary decides to go to the grocery store and get things for supper—the supper Patrick said he did not want, but the dinner she pretends she is making because he was too tired to go out to a restaurant (their habit on Thursdays) after coming home.
Suspense begins to build as Mary tells the police that she left Patrick to buy some things for supper; one of the policemen goes to the store to speak with the grocer:
In fifteen minutes he was back with a page of notes, and there was more whispering, and through her sobbing she heard a few of the whispered phrases-”...acted quite normal...very cheerful...wanted to give him a good supper…peas...cheesecake...impossible that she...”
This presents the idea that Mary was under some suspicion at least as a technicality because the policeman goes to corroborate her story, and they assume they can dismiss her from a list of likely suspects.
There may also be some suspense when they search for the murder weapon. The reader knows how Patrick was killed. Even when Mary hit him, the narrator (using foreshadowing) notes:
She might just as well have hit him with a steel club.
The fact that the police remain at the house, searching for clues provides suspense in that until they are satisfied that there is nothing that might implicate Mary in the murder, Mary is not yet safe. As they search for the weapon, which the police note could have been taken by the murderer or hidden on the premises, the audience is left to wonder if Mary will be caught. The importance of the leg of lamb is evident as Jack Noonan explains the integral nature of the weapon to their investigation:
“It’s the old story,” he said. “Get the weapon, and you’ve got the man.”
The last bit of suspense occurs when Noonan reminds Mary that the lamb is still cooking. She pretends to be startled and asks him to turn the oven off. There is never the slightest indication that Jack Noonan suspects anything about the once-frozen leg of meat now finished cooking. To bring the suspense to an end, Mary tearfully invites the men to eat dinner with her. When they agree and eat their fill, the police (of all people—how ironic) have unknowingly disposed of the evidence, and Mary's freedom is assured.
Mixed in with the suspense and irony is black humor, where the grotesque circumstances of Patrick's death provide not only suspense, but comedy as well, as the men eat the only thing to tie Mary to the crime. As they finish their meal, the men discuss the case:
“That’s the hell of a big club the gut must’ve used to hit poor Patrick,” one of them was saying [...] "Whoever done it, they’re not going to be carrying a thing like that around with them longer than they need.”
One of them belched.
"Personally, I think it’s right here on the premises.”
“Probably right under our very noses."
How right they are!