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 20, Sheppard uses a simile to describe Poirot's approach to the investigation. The simile relies on a visual image:
“You know,” I said, throwing down the pincers I was holding, “it’s extraordinarily intriguing, the whole thing. Every new development that arises is like the shake you give to a kaleidoscope—the thing changes entirely in aspect.
A kaleidoscope is an instrument, usually a tube, with beads or pieces of colored glass at one end and angled mirrors positioned around the inside. A viewer can look through the open end of the instrument and see a vivid, tessellated pattern created by reflections of reflections of the colored glass. Turning or shaking the kaleidoscope creates an entirely new pattern by changing the arrangement of the colored glass.
Sheppard conjures the image of a kaleidoscope and compares the investigation to it. Poirot is walking Sheppard through a very different investigation than the one the police have conducted. Whereas the police have sought a straightforward, objective story of what happened, Poirot is after a collection of subjective accounts. He hopes to piece together all these accounts into an objective narrative, but for now each piece of testimony is like a new arrangement of colored glass, casting an entirely different image onto the viewer's eye.
Sheppard is still trying to get away with murder at this point. He emphasizes the way in which each development changes the image in the "kaleidoscope" of the investigation. He passes himself off as merely intrigued, but it is easy to imagine that he thinks the pieces are scrambled enough to conceal the truth forever. What he does not note is that while a kaleidoscope can cast seemingly infinite patterns, there is a finite number of glass pieces resting at the bottom. Poirot's goal is to look at as many patterns as he can and deduce what really lies beneath them all. Sheppard believes that seeing more patterns opens more possibilities, but Poirot knows that each new pattern is another clue to the truth.
In Chapter 23, Sheppard uses a simile comparing Caroline to a dog. This simile is part of a broader motif of animal comparisons Sheppard uses to describe Caroline:
We went off, leaving Caroline rather like a dog who has been refused a walk, standing on the front door step gazing after us.
The simile does not just serve as a neutral descriptor. Rather, it establishes a power dynamic in Sheppard and his sister's relationship. Caroline "has been refused a walk." Like a dog who is forgotten at home, she is not allowed to go outside except at the invitation of her person. There is an underlying sense that if Sheppard did bring Caroline out of the house, he would have to keep her on a figurative leash to keep her from bothering people or putting herself in harm's way. Sheppard seems to take a cruel delight in his sister's tragic "gaze" and in the empowering act of refusing her an invitation.
When Sheppard first introduces Caroline, he compares her to Rikki-Tikki-Tavi, a mongoose from a Rudyard Kipling story who is too curious and adventurous for his own good. Throughout the novel, he frequently uses these animal comparisons to insult Caroline. The comparisons provide comic relief, but more often than not they also have overtones of misogyny. The mongoose comparison, for instance, is a critique of Caroline's interest in gossip, which is a pursuit often associated with women. The dog simile in Chapter 23 likewise puts Caroline "in her place," so to speak, as a woman. It emphasizes that she belongs inside the house, not out in the world. In also denigrates her and other women by suggesting that women and dogs are alike in this regard.
Christie is generally a lighthearted writer, and she lets Sheppard have some fun at Caroline's expense. She even seems to expect that the reader will laugh at some of these jokes as well. Still, the fact that it is the killer who makes all these jokes suggests that indulging in them regularly is not a very honorable way to conduct oneself.
