If a criminal were a psychologist.

If a criminal were a psychologist, what a criminal he could be! said Mr Parker Pyne. His voice dwelt lovingly on the idea. 'When you think that of ten people you meet, at least nine of them can be induced to act in any way you please by applying the right stimulus'.
'Oh, explain that!' cried Carol.
'There's the bullyable man. Shout loud enough at him - and he obeys. There's teh contradictory man. Bully him the opposite way from the way in which you want him to go. Then there's the suggestible person, the commonest type of all. Those are the people who have seen a motor, because they are told a man has been stabbed; or who will have heard the pistol if they are told a man has been shot.'

The Pearl of Price
Parker Pyne Investigates
Agatha Christie (1934)
HarperCollins