The great magic of method acting comes when actors steep themselves in reality as much as possible: they seek to replicate real emotions by seeking to perfectly mimic the conditions that created them. Actors launch themselves into the lives of the characters they're set to play, seeking an added layer of realism that blindly playing pretend simply can't compete with, which is precisely why the plaudits tend to come to them so loudly.
But sometimes knowing method is only partly successful, and it's even better to have the real emotions and reactions of actors caught on screen. That can come in the form of one-take improvisations, accidents left in because of how good they look or - rather more sinisterly - actors are literally tormented by directors looking for extra, or their real pain is kept in.
And while you might view the idea of a director purposefully torturing their stars to get a more legitimate reaction in the can, it comes with a long and storied history, with names as impressive as Hitchcock, Cameron, Scott and Kubrick all implicated.
Sometimes, there's just no competing with real reactions...