Oooh, that is a fabulous question.

For Eowyn, it's voice. Tolkien's voice is very specific and the army of angry fans that would descend upon me if I screwed it up is large.

For Nikita, it's conveying her eternal calculation. There's always something else going on behind whatever face she's choosing to show the world.

For Sam, it's making him not a caricature. Since we only see him from Sookie's point of view in his canon, he could easily become flat and "Well, gawsh dangit, howdy do".