Polanksi played mind games with Nicholson; 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.
Private eye Jake Gittes (Jack Nicholson) has an unwelcome encounter with two hoods at the reservoir. One of them (director Roman Polanski) menaces Gittes with a switchblade, in Polanski’s masterful Chinatown (1974)