Captain Arthur Hastings and Evelyn Howard’s conversation regarding detective work and the difference between reality and fiction is full of both foreshadowing and situational irony:
“Like a good detective story myself,” remarked Miss Howard. “Lots of nonsense written, though. Criminal discovered in last chapter. Every one dumbfounded. Real crime—you’d know at once.”
“There have been a great number of undiscovered crimes,” I argued.
“Don’t mean the police, but the people that are right in it. The family. You couldn’t really hoodwink them. They’d know.”
In the passage above, Evelyn confidently insists that murder mysteries are unrealistic even if they are entertaining, claiming that in real life crimes are much more easily solved. This assertion is an ironic bit of foreshadowing, as Christie humorously pokes fun at the genre of her own novel. Evelyn’s remarks on the absurdity of the detective story structure—the grand final reveals and “dumbfounded” reactions—lay out the exact sequence of plot events that later happen in Christie’s own novel. Further, Evelyn’s declaration that it would be impossible to effectively “hoodwink” those involved in the story (thereby drawing out the mystery) essentially functions as a dare to the reader, inviting them to be on the lookout for clues. As one of the two villains of the story, along with Alfred Inglethorp, Evelyn’s comment regarding the relative mundanity of real-life crime foreshadows her own involvement in Emily's murder by priming Hastings and the other residents at Styles Court to rely on their first, impulsive instincts.
Evelyn Howard’s argument with John Cavendish midway through Chapter 5 regarding her belief in Alfred Inglethorp’s guilt is steeped in layers of situational irony, particularly considering Hastings’s own musings as he recounts these events from memory:
It occurred to me very forcibly at that moment that to harbour Miss Howard and Alfred Inglethorp under the same roof, and keep the peace between them, was likely to prove a Herculean task, and I did not envy John. I could see by the expression of his face that he fully appreciated the difficulty of the position.
In the passage above, Hastings recognizes Evelyn’s distaste for Alfred in light of her verbal insistence on the man’s guilt. It is this perceived animosity that leads Hastings to assume that John Cavendish will have a hard time balancing Evelyn and Alfred together. However, Hastings's observation that Evelyn and Alfred are enemies who find it difficult to civilly converse is deeply ironic in light of the later revelation of their amorous, murderous affair. The allusion he makes here to Hercules—the Greco-Roman mythological figure whose fame is owed to his divine, god-given strength—reflects how deeply he believes the pair dislike each other and how well their ruse is working on him.
As The Mysterious Affair at Styles comes to a close, Hastings demands that Poirot explain the logic behind his mysterious actions throughout the novel—especially his decision to keep Hastings in the dark regarding the identity of the culprit. While Poirot indulges this request, he takes some time to examine the antagonists’ central construction of the mystery. In doing so, Poirot uses verbal irony to present Inglethorp’s line of thinking as he developed his plan:
Because, mon ami, it is the law of your country that a man once acquitted can never be tried again for the same offence. Aha! but it was clever—his idea! Assuredly, he is a man of method. See here, he knew that in his position he was bound to be suspected, so he conceived the exceedingly clever idea of preparing a lot of manufactured evidence against himself. He wished to be suspected. He wished to be arrested. He would then produce his irreproachable alibi—and, hey presto, he was safe for life!
As Poirot explains how Inglethorp worked out his plan to avoid discovery in the passage above, he expresses a degree of admiration for the level of method and detail the antagonist put into his alibi. However, the very fact that Poirot is able to figure out what happened proves that Inglethorp's planning was not thorough enough to avoid detection. When Poirot says, "[...] hey presto, he was safe for life!", he is using verbal irony—after all, Inglethorp is not, in fact, "safe for life." The flimsy staying power of Inglethorp’s supposedly “irreproachable” alibi makes it clear that Poirot is speaking facetiously, ultimately using irony to emphasize Inglethorp's misguided belief that he would be able to pull off this plan.
