An LA private investigator called JJ 'Jake' Gittes, played by Jack Nicholson, has been hired to spy on the chief engineer of the city water department. The mystery is why gallons of water are being sluiced from reservoirs at night – in the middle of a drought.

So here is Gittes nosing round what is supposedly the Lake Hollywood Reservoir, built by Water Commissioner William Mulholland in 1925. He's seen by a security guard called Claude Mulvihill (Roy Jenson). Walking beside him is a sinister character with a rolling stride played by the director Roman Polanski in a cameo. Jenson grabs Nicholson. Polanski whips out a flick knife and sticks it up Nicholson's nose. "You're a very nosy fellow, kitty cat," says Polanski.

When repeatedly asked about this scene in later years, Polanski and Nicholson, bored with giving the same answer, often pretended that what is shown on screen, over the next few seconds, really happened. Polanski slits Nicholson's nostril with his knife and the blood covers his face. How was it done?

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The filming of Chinatown had been a fractious business, with Polanski flying into rages with Nicholson. This tension freights the scene, which was filmed at Point Fermin in south LA. When Nicholson says, as the diminutive director ambles towards him, "Hello, Claude, where'd you get the midget?", there's a personal sting to the remark.

In turn, Polanksi played mind games with Nicholson; the latter knew that Polanski had designed the prop knife himself, and that if the hinge faced the direction he was going to wrench it, the knife would fold safely back into the handle. If it wasn't correctly placed, it would genuinely cut Nicholson's nose. On each take, Polanski would flip the knife over and over until it wasn't clear whether it was safe. That's genuine fear and apprehension in Nicholson's eyes.

In fact, it's clear how the trick was done if you look closely at Polanski's hand holding the knife, which would have had a reservoir of fake blood in the handle and a tube out of sight to deliver it. Just before he jerks the knife away, you see his thumb move slightly to the left to activate the button to release the blood.