In Chapter 3, Sheppard tries to distract Caroline from her gossip about Ackroyd, Flora, and Ralph by casually mentioning that he has spoken to the new neighbor, "Mr. Porrot." Sheppard uses a simile and an idiom popular in mystery novels to describe Caroline's reaction:
Caroline visibly wavered for a second or two, much as if a roulette ball might coyly hover between two numbers. Then she declined the tempting red herring.
A "red herring" is a clue or avenue of inquiry that is ultimately a dead end. It comes from an 1807 story by the political writer William Cobbett, in which he claimed to have used a fragrant smoked fish to misdirect hunting hounds from the trail of a rabbit. By the time Christie was writing, it was an idiom that often appeared in mystery writing. It referred more broadly to misleading evidence, sometimes but not always planted deliberately. A good detective, like Hercule Poirot, is good at filtering out red herrings and pursuing the truly useful clues. By referring to his own mention of "Mr. Porrot" (Poirot) as a red herring, Sheppard suggests that he is in a battle of wits with his sister. She wants to gossip about Mr. Ackroyd and his family, and Sheppard wants to see if he can get her onto a different topic by mentioning an intriguing new neighbor. Caroline does not rise to the bait he has set for her. She proves herself to be a canny detective.
A closer look at Sheppard's simile reveals that the battle between Sheppard and Caroline may be more than a friendly battle of wits. The game of roulette revolves around a wheel that has numbers printed all around the outside and a circular track that lines up with the numbers. A player bets on a number or set of numbers; then a dealer spins the wheel in one direction and sends a ball in the opposite direction around the track. When the ball loses momentum, the number it lands on determines the outcome of the bet. Sheppard describes his sister like "a roulette ball" that "coyly hover[s] between two numbers." She finally decides not to go for Sheppard's "red herring" and instead stays on the topic of Ackroyd and his family. Caroline is not like the gambler in this simile, but rather like the ball that determines the gambler's fortune. As is revealed at the end of the novel, Sheppard will murder Roger Ackroyd this very night. Caroline's keen attention to the neighborhood gossip has the potential to ruin Sheppard's plan to get away with murder. When she chooses not to take Sheppard's "red herring," she demonstrates that he needs to watch out for her.
In Chapter 19, Inspector Raglan uses a pair of idioms as he informs Dr. Sheppard that Poirot has a mentally ill nephew who has been institutionalized nearby. This moment foreshadows the climax of the novel:
["]He’s got a nephew who’s quite off his crumpet.”
“Poirot has?” I said, very surprised.
“Yes. Hasn’t he ever mentioned him to you? Quite docile, I believe, and all that, but mad as a hatter, poor lad.”
“Who told you that?”
Again a grin showed itself on Inspector Raglan’s face.
“Your sister, Miss Sheppard, she told me all about it.”
"Off his crumpet" and "mad as a hatter" are both cliché (and insensitive) ways of saying that someone is mentally ill. In fact, Poirot does not have a mentally ill nephew. As the detective reveals at the end of the novel, he has made up the story about his nephew in order to get information out of Caroline. He suspects Dr. Sheppard of hiding Ralph at a nearby institution and tells Caroline about his fake nephew so she will tell him the names of any institutions her brother has visited recently.
The nephew is both a red herring and a Chekhov's gun. On the one hand, Poirot's story about his nephew leads his neighbors to the wrong conclusion, keeping them from realizing that he knows where Ralph is or that he suspects Dr. Sheppard of involvement in his disappearance. Poirot is able to keep Sheppard's trust for far longer than he would have been able to if he had openly accused him of hiding Ralph as soon as he suspected it. By visiting Ralph under the pretense of visiting his nephew, Poirot is also able to conceal from others that Ralph has been located. This concealment allows him to plant the newspaper ad that helps him draw out Ursula's confession that she and Ralph are married.
On the other hand, this mention of Poirot's nephew is a Chekhov's gun in that it eventually blows up in Sheppard's face. Believing that Poirot is really visiting his nephew at the Larches lulls Sheppard into a false sense of security. He lets Poirot read his notes on the case and keeps working with him long past the point when he is the main subject of the detective's investigation. Inventing the story about a nephew is what finally allows Poirot to find proof that Sheppard has been lying by omission.
A reader can easily miss the signs that the nephew is all that relevant, but there are some subtle indicators in this scene that Poirot might be up to something. For one thing, Raglan's use of idioms to describe the nephew allows him to be very vague about this relative's condition. There is no developed backstory for him, which is a bit odd given that Caroline's sole mission in life is to "find out" all she can about the neighbors. The very fact that Caroline is the person Poirot first told about his nephew is also suggestive. He knows she is a gossip, so he must have told her with the understanding that she would spread the information far and wide, until eventually it reached Dr. Sheppard.
