In
Far And Away, when Nicole Kidman peeks under the bowl covering Tom Cruise's genitals, she didn't look surprised enough, so director Ron Howard had Cruise take off his underwear without Kidman's knowledge.
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The iconic Japanese director Akira Kurosawa was a perfectionist, and expressions of fear are notoriously difficult to reproduce. In the climatic death scene of his film
Throne of Blood, the arrows that miss aren't props.
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In
Straw Dogs, to get the perfect "shocked reaction" from the villagers when the main character walked into the pub, the director had them walk in without any trousers on. It worked.
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In the
Red Dwarf episode "D.N.A.", Lister (played by Craig Charles) is handed a photo of a man's genitals and reacts accordingly. During rehearsal, the photo was always something mundane, but when they actually shot the scene, Craig was given a photo of a guy's crotch without warning.