Karl is such a good method actor and he doesn't even know it. I heard an interview where he said what he did wasn't acting at all. He just put himself in the character's shoes and acted like they would, which is method acting.
That scene where he gets rid of the greedy lady who was only interested in her mum's valuables took a lot out of Karl. He was genuinely angry and was shaking afterwards. There is a clip of him talking about it ...somewhere!