The Dark Knight
For the first four days of production, Christopher Nolan put cast and crew under a movie boot camp comprising eight films whose tone he wanted to emulate. In chronological order, these were: King Kong, Citizen Kane, Cat People, Stalag 17, Black Sunday, A Clockwork Orange, Heat and Batman Begins.
The Shawshank Redemption
Morgan Freeman's line, "Maybe it's 'cause I'm an Irishman" was not written to be a joke. In Stephen King's novella, 'Red' really is Irish.
The Godfather
Although popular myth suggests that Marlon Brando padded his cheeks with cotton wool to play Vito Corleone, he did so only for the audition. Before the actual filming began, he had a mouthpiece specially created by a dentist.
Donnie Darko
Gary Jules' cover of Mad World - later a UK Christmas #1 single - was a last-minute replacement when Richard Kelly couldn't get the rights to his first choice, U2's MLK.
The Usual Suspects
The line-up scene was meant to be treated serious, but the actors' corpsing led an exasperated Bryan Singer to change tack and (unlike Kubrick in Doctor Strangelove) use the funniest takes.
North by Northwest
Cary Grant was actually older than his on-screen mother, Jessie Royce Landis.
Oldboy
Min-sik Choi wolfed down four live animals during the notorious octopus eating scene. The actor, a Buddhist, then had to go and pray in atonement.