Ron Rosenbaum reveals documentarian Errol Morris' trick, which he picked up while working with a private eye:
It wasn’t a blackjack-, brass knuckles-type thing. “It went like this,” Morris explained. “He’d knock on a door, sometimes of someone not even connected to the case they were investigating. He’d flip open his wallet, show his badge and say, ‘I guess we don’t have to tell you why we’re here.’ “And more often than not the guy starts bawling like an infant, ‘How did you find out?’” And then disgorges some shameful criminal secret no one would ever have known about otherwise.
The Dish 