From Ron Rosenbaum’s Smithsonian profile of the filmmaker Errol Morris:

I wanted to tell you about his private-eye trick, which he learned from a hard-bitten partner.

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.

I have a feeling about why Morris likes this. There’s the obvious lesson—everybody’s got something to hide—and then there’s the subtle finesse of the question: “I guess we don’t have to tell you…” No water-boarding needed, just an opening for the primal force of conscience, the telltale heart’s internal monologue. It’s one of those mysteries of human nature that private eyes know and Morris has made his métier.

You ever see Morris’s stunning documentary, “Mr. Death: The Rise and Fall of Fred A. Leuchter, Jr.”? It’s one of the creepiest, but most compelling, things I’ve ever seen. Really, see it if you can. It’s a portrait of this nebbishy guy from Massachusetts who becomes a self-taught expert in the mechanics of prison execution. He gets drawn into the world of Holocaust denial when he’s asked to examine Auschwitz, to see if mass execution of Jews could have happened there. In Fred A. Leuchter, Jr.’s expert opinion, it could not have. He becomes a celebrity on the neo-Nazi, Holocaust denier circuit, and basically destroys his life. All throughout, he really has no idea what he’s done, or why this has happened to him. He comes across as sincerely clueless — which is why his story is so eerily compelling.

You realize, of course, that Morris’s real subject is not the Holocaust, or Holocaust denial, but rather how we know what we know, and how something as common as pride, and the need to be respected, can lead us badly astray. This comment from a historian in the film cuts to the heart of Morris’s subject:

Leuchter is a victim of the myth of Sherlock Holmes. A crime has been committed. You go to the site of the crime and with a magnifying glass you find a hair, or you find a speck of dust on the shoe. Leuchter thinks that is the way reality can be reconstructed. But he is no Sherlock Holmes. He doesn’t have the training. It was not that he brought any experience, the specific experience needed to look at ruined buildings. The only experience he had was design modifications for the Missouri gas chambers in Jacksonville.

In the film, you can see that Leuchter is not motivated by hatred of Jews. He is motivated by an unshakable faith in himself, and by the attention he gets from the Holocaust deniers. Finally, someone recognizes his genius! The fact that Fred Leuchter is not a snarling, evil figure, but an ordinary, happy-go-lucky geek trapped by his own intellectual pride is what makes this movie so strange and compelling.