For me, it always comes down to the point in time where the story is set, the intimacy of the characters, and, of course, canon reference.

The classic examples that come to mind for me are Stargate SG1 generally: who refers to whom by what name is very dependent on whose POV I'm coming from. It always made writing AUs rather interesting.

Or are you referring to how I talk about the characters when in discussion with other fans?