If you go into a bank or a shop and you want them to believe that you're going to shoot them, that's an acting exercise. If you want to turn to someone else who's as tooled up as you are and persuade them to put their knife down because you'll use your knife, that's an acting exercise. Nine out of 10 delinquents are frustrated actors.
Every film I've ever worked on, and that includes 'Braveheart' and 'Trainspotting,' I've always witnessed a director having a breakdown. Every director will have a day, without exception, where they just can't do it anymore, they don't know what to say to their cameraman, their cast. It's the sign of real, physical exhaustion.