The guy who played Karl Tanner...

Quite frankly, I thought the guy who played Karl Tanner (Burn Gorman) was a big old ham, just a little facial hair away from being an old-fashioned monologuing moustache-twirling overacting camp villain. No subtlety at all.

Iwan Rheon, on the other hand, played it big enough to command attention when it was called for, but not toooooo big. And he really did try to make the character more entertaining and interesting than he was in the books, because face it, in the books and script, Ramsay Bolton was nothing more than a collection of sadistic fantasies. The writers all sat down and thought of the grossest most violent acts they could thing of, heaped them all in a big pile, and called the resulting mess "Ramsay". Rheon tried to make the guy into a person, a person with feelings and motives and an awareness that he needed to act normal or even courtly at times. Rheon did what good actors do, make a character written as one-dimensional into a three-dimensional human being. Or at least he tried, I can't say he completely succeeded, but then all the writers were against him.