25. M*A*S*H (1970)
Actor who stripped down: Sally Kellerman
Why they're fully nude: An innocent shower turns into an all-out prank.
Spontaneity and authenticity were the two things that made a Robert Altman set "A Robert Altman Set," with not even the actors knowing whether they were being filmed at any given moment. Which meant that one always needed to be on his or her toes. Sally Kellerman learned that lesson the hard way when she voiced her lack of enthusiasm for engaging in a split second of full-frontal nudity.
As the uptight "Hot Lips" Houlihan, Kellerman's natural assets come into question in the age-old "carpet versus drapes" debate. Her colleagues devise a simple plan for determining the answer: raise the shower flap while she's scrub-a-dub-dubbing away. Easy enough. Except Kellerman wasn't having any of it.
Each time the curtain was raised, Kellerman would scramble to the ground before the camera could capture the goods. Uh-uh—not authentic enough for Altman. So on the final take, the ever-inventive director and Gary Burghoff (who played "Radar" O'Reilly) dropped their own pants before the curtain was lifted, stunning Kellerman just long enough to get the shot Altman wanted.