I like the fact that it's so character-driven. It's clear why Todd, with that 'intensity', that dogged attachment to what he believes to be his duty, that tendency to spin around and around in his thoughts without getting anywhere new, is fascinated by the mercurial, flexible, opportunistic and quick-thinking Karl. Even Robbie is not just a jealous violent lover. Karl's dodging and feinting were clearly driving him crazy just as much as Todd, weren't they? In that short scene where he is first introduced to Todd, Karl in turn acts wronged simple dignity, makes Robbie smile by flattering his physical fitness, and warns him he's not intimidated, he'll be coming home when he's good and ready. Robbie can hardly be satisfied by that clever mix, and he isn't. Karl is a genius at making Todd and others feel safe when he wants that ('just because you kissed a bloke doesn't mean you're gay', and then somehow making Todd make secure spending so much time with an out gay man even with Nick always around). And then, making them feel at risk by teasingly drawing back ('Well, that's the question, isn't it?') Also ready at a moment's notice to drop the joking and put on his best manners.
Yet he is fascinated by Todd as a complementary half too.
