Generally in cultural products like movies, television, books, and even video games, characters who appear to be one thing end up being full of contradictions, acting more like their opposit. It is a common held belief that said contradictions are what make a character more "human."
So when you see a good hearted guy like Blackwall, with a vague name and talking vaguely about things and talking about honor and greater good, he's gonna have something dark and seedy lurking under the surface. Luckily, it was only murder (come on, your character is a murderer too) and not something real creepy. Or when some Elf shows up at the right point in time knowing a whole hell of a lot and having rare powers and rare knowledge, he's not just gonna be an apostate.
Cullen, on the other hand, seems all aloof and job focuses, so you know he's gonna be pure, unadulterated vanilla ice cream with whip cream and marshmallows in the romance arc.
Little tip about who to pick when romancing someone. Kept me from making the mistake with Anders back in DA2 my first play through.