Murder on the Orient Express
Poirot shook his head violently.
"That is just it – it is impossible – quite impossible – that an honourable, slightly stupid, upright Englishman should stab an enemy twelve times with a knife! Do you not feel, my friends, how impossible it is?"
"That is the psychology," said M. Bouc.
"And one must respect the psychology. This crime has a signature and it is certainly not the signature of Colonel Arbuthnot." (2.8.118-121)
"I look first at my witness, I sum up his or her character, and I frame my questions accordingly. Just a little minute ago I am asking questions of a gentleman who wants to tell me all his ideas on every subject. Well, him I keep strictly to the point. I want him to answer yes or no, this or that. And then you come. I see at once that you will be orderly and methodical. You will confine yourself to the matter in hand. Your answers will be brief and to the point. And because, Mademoiselle, human nature is perverse, I ask of you quite different questions. I ask what you feel, what you thought. It does not please you this method?" (2.11.32)
"Did I not tell you that I was, like you, a very puzzled man? But at least we can face our problem. We can arrange such facts as we have with order and method." (2.13.16)