"And then?" "And then," said Poirot. "We will talk! Je vous assure, Hastings - there is nothing so dangerous for anyone who has something to hide as … - Agatha Christie

"And then?"
"And then," said Poirot. "We will talk! Je vous assure, Hastings - there is nothing so dangerous for anyone who has something to hide as conversation! Speech, so a wise old Frenchman said to me once, is an invention of man's to prevent him from thinking. It is also an infallible means of discovering that which he wishes to hide. A human being, Hastings, cannot resist the opportunity to reveal himself and express his personality which conversation gives him. Every time he will give himself away."
"What do you expect Cust to tell you?"
Hercule Poirot smiled.
"A lie," he said. "And by it, I shall know the truth!"