In Chapter 7, Poirot and Sheppard examine the murder scene. Poirot uses logos to explain why he is interested in the odd positioning of a chair in the room, foreshadowing the way he will out-think Sheppard in the end:
“Raymond or Blunt must have pushed it back,” I suggested. “Surely it isn’t important?”
“It is completely unimportant,” said Poirot. “That is why it is so interesting,” he added softly.
The chair was reportedly pulled out from the desk at an odd angle. Sheppard insists that it must not be very important—after all, Ackroyd was murdered across the room from the chair. A reader who knows that Sheppard is the killer can see in retrospect that he is trying to use logos of his own to get Poirot to ignore the chair. Sheppard knows exactly why it was in a strange position. He pulled it out to block the dictaphone on the desk from sight. When he killed Ackroyd, he set up a dictaphone with a timer to make it sound as though Ackroyd was alive and speaking to someone after he was already dead. This way, Sheppard has given himself an alibi. He hid the dictaphone so that when people first arrived at the murder scene, he would have a chance to retrieve it before anyone saw it.
Poirot is one step ahead of Sheppard. His process as a detective involves gathering as much information as he can in order to piece together not the most obvious story, but the truest one. Poirot points out here that seemingly unimportant details often become important once they are contextualized by other unimportant details. Sheppard's attempt to steer Poirot away from the chair's significance backfires, instead making it more "interesting" to him.
In Chapter 14, Sheppard speaks with Ursula Bourne and tells her that the murder must have taken place after 9:45 at night. Ursula subsequently demonstrates a strange interest in Ralph Paton, foreshadowing the revelation at the end of the novel that they are secretly married:
“Excuse me, sir, is there any news of Captain Paton?”
I shook my head, looking at her inquiringly.
“He ought to come back,” she said. “Indeed—indeed he ought to come back.”
She was looking at me with appealing eyes.
“Does no one know where he is?” she asked.
“Do you?” I said sharply. She shook her head.
“No, indeed. I know nothing. But anyone who was a friend to him would tell him this: he ought to come back.”
Ursula seems relieved to hear that Roger Ackroyd was murdered after 9:45. This new timeline seems to clear Ralph of suspicion because he can't have been at the house after 9:45. As much evidence as is stacked against him, a credible alibi proves that he is innocent. Ursula does not say anything in this conversation to reveal outright that she is married to Ralph. Her concern for him could be born out of mere empathy. However, her repeated insistence that "he ought to come back" is a bit mysterious. Sheppard mistakes Ursula's curious interest in Ralph for a guilty conscience. Sheppard does in fact know where Ralph is, so he is primed to assume that anyone acting squirrely on the subject of the missing man knows something about his whereabouts as well.
At the end of the novel, when it turns out that Ursula and Ralph are married, all the pieces fall into place. This is why Ursula insisted that Ralph should come back once it seemed clear that he did not murder his stepfather. This is also why there was a woman's wedding ring in the pond bearing an inscription from "R."
In Chapter 17, Poirot describes the psychology of the person he believes killed Roger Ackroyd. He employs a metaphor to describe the mindset of a blackmailer-turned-murderer, foreshadowing his eventual accusation of Sheppard himself:
Here is a chance of money—a great amount of money. He wants money—he desires it—and it is so easy. He has to do nothing for it—just keep silence. That is the beginning. The desire for money grows. He must have more—and more! He is intoxicated by the gold mine which has opened at his feet. He becomes greedy. And in his greed he overreaches himself.
Poirot describes a person who is not particularly evil but who stumbles into a combination of financial desperation and sinister opportunity. As he realizes that he has access to easy money through blackmail, he keeps wanting more and more. The metaphorical "gold mine" he finds at his feet becomes an obsession. The metaphor recalls recent gold rushes in the U.S., in which some people got rich but most people risked and lost everything they had. Similarly, the blackmailer's greed leads him to risky behavior from which he can't bounce back. Now more desperate than he was in the beginning, the blackmailer turns to murder in a last ditch effort to save himself.
While Poirot does not specify who he is talking about and lets Caroline and Sheppard believe he means Ralph Paton, Poirot's profile foreshadows the revelation that Sheppard is the killer. As the reader comes to find out by the end of the novel, Sheppard is a normal man who was drawn into blackmail and finally murder because he ran into money problems. Even if the reader does not yet suspect Sheppard, Poirot's speech foreshadows the fact that the killer will likely be someone who seems trustworthy. In retrospect, it also demonstrates that Poirot suspects Sheppard even while he seems to be cooperating with him on the case.
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.
