Actors who stripped down: Just about everyone
Why they're fully nude: It's not called Fully-Clothed Holocaust
First things first: You can't really have a cannibal movie without a flash of flesh or two. But if any movie ever proved that nudity is NOT always sexy—and is probably best in small doses—it's Cannibal Holocaust, Ruggero Deodato's infamous version of a National Geographic documentary gone terribly, horribly, impossibly wrong. The nudity here is rampant, and part of the film's statement on the struggle between civilized and uncivilized societies (which the director intended as a slur against the Italian media).
The original "found footage" movie (you're welcome, The Blair Witch Project), Deodato may have been striving for a bit too much authenticity when shooting Cannibal Holocaust, mixing professional actors with real Amazonian tribesmen and women. The director was actually arrested on obscenity charges 10 days after the film's release, when the finished product was believed to be a snuff film, with rumors swirling about some of the on-camera deaths being legitimate murders. (The director had to track down a few of the actors and bring them to court to prove that they were indeed still alive.)
One aspect that was real: the killing of several animals, many of them by machete. Those charges did stick and succeeded in preventing this film to ever be widely released, leading this Holocaust to be named one of the most controversial flicks of all time.