Actor who stripped down: Sissy Spacek
Why they're fully nude: High school. Gym class. Showers.
Back before he was making crap like Snake Eyes and Mission to Mars, Brian De Palma was a master of suspense, creating memorably unsettling tones in movies like Sisters and Carrie, the first big-screen adaptation of a Stephen King story. And before women's locker rooms became a place for peeping (see Porky's), De Palma made them a place for higher learning. About womanhood and what happens when you cross a teen with telekinetic powers.
De Palma aims for uncomfortable in the opening scene of Carrie and scores a direct hit as soft music and lighting give way to absolute terror when gawky teen Carrie White (Sissy Spacek, in her first of six Oscar-nominated performances) mistakes the onset of her menstrual cycle for imminent death and frantically charges her classmates in all her naked terror.
Like all good teen movies, this one's got its fair share of mean girls, who teach Carrie the facts of life by pelting her with a stockpile of tampons. What her peers fail to realize is that anger is the trigger for Carrie's psionic powers...so they're all going down.