At the end of Three Act Tragedy, after the murderer has been revealed and some after-discussion is happening, a character asks Poirot an interesting question:

“You’ll excuse me—” said Mr. Satterthwaite.

“Yes, there is some point you wanted explained to you?”

“There is one thing I want to know.”

“Ask then.”

“Why do you sometimes speak perfectly good English and at other times not?”

Poirot laughed.

Ah, I will explain. It is true that I can speak the exact, the idiomatic English. but, my friend, to speak the broken English is an enormous asset. It leads people to despite you. They say—a foreigner—he can’t even speak English properly. It is not my policy to terrify people—instead I invite their gentle ridicule. Also I boast! An Englishman he says often, ‘A fellow who thinks as much of himself as that cannot be worth much.’ That is the English point of view. It is not at all true. And so, you see, I put people off their guard. Besides,” he added. “it has become a habit.”

Three Act Tragedy was published in 1935, after Murder On the Orient Express and before Death in the Clouds. It is set quite late in Poirot’s life; he was, at this time, retired.

This habit of Poirot’s solves a problem that all detective writers face: a lot of people don’t like to talk to detectives. There are different solutions to this problem; Poirot in general likes to set people at ease and make them think that the easiest way to deal with him is just to humor him. This was taken even further by Columbo, many years later, but it certainly makes sense as an approach.

It also makes sense that Poirot decided to turn his disadvantage—the famous dislike of the English for foreigners, especially for French-speaking ones—into an advantage.