Shia LaBeouf in Fury
LeBeouf wanted his role to seem as believable as can be, and so portraying a soldier on the battlefield, he cut his cheek and kept the wound open during the entire shooting of the film, to make his suffering real.
He didn’t settle for just cutting his cheek (was it not painful enough?), but he also pulled a Nicholas Cage and pulled out one of his teeth. And if that’s not enough, he pulled a Halle Berry and didn’t shower during the month he spent on set.