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 18, Poirot tells Sheppard over dinner that he suspects something about Charles Kent but does not specify what. Sheppard uses a metaphor to describe Poirot's process of investigation and deduction:
I know now that the whole thing lay clearly unravelled before him. He had got the last thread he needed to lead him to the truth.
But at the time I had no suspicion of the fact. I overestimated his general self-confidence, and I took it for granted that the things which puzzled me must be equally puzzling to him.
Sheppard describes the investigation as a tangle of thread. Poirot has spent the investigation picking loose threads out of the pile, sorting them and finding out where each one leads. Most have led nowhere; Poirot has persisted in trying to find the end that leads to the truth at the center of the mess. Sheppard reflects in retrospect that at this moment, when he expresses his suspicion over Charles Kent, Poirot has the very thread he has been looking for. But at the time, Sheppard still thinks he is getting away with murder. He does not realize that Poirot has been investigating him and that he is now the prime suspect.
