The issue with "That's what my character would do" is when people use it as an excuse for being an (bleep). You might be making character-appropriate choices, but you designed that character to be an (bleep), so your characters (bleep)-ness is your fault. Of course that doesn't mean your character has to integrate completely without friction into the party – some friction is fun! But any disruptiveness on part of your character is your responsibility.
I dissociate the playing of a [bleep] character from being a [bleep] at the table and somewhat expect others to do likewise.
I mean, an exchange like this is, for me, excellent:
Player 1, shrieking as Celesta (PC): "You stole my wand! Prepare to die, thief!" <
loads up for a killing spell>
Player 2, yelling as Dodger (PC): "Cast that [bleep] at me, will ya? Not bloody likely!" <
throws dagger at Celesta, hitting and interrupting her spell>
Player 1, out of character: "Hey, try some of these new-flavour chips, they're great!"
Player 2, out of character: "Thanks. How's your beer doing?"
Player 1, shrieking as Celesta (PC): "Gaaah! Now interrupt
this!" <
pulls out another wand and aims it at Dodger>
... etc.
Meanwhile Players 3 and 4, probably laughing their damn-fool heads off out-of-character, have their characters react accordingly in-character even if such reaction consists merely of diving for cover. And the DM sits back, puts his feet up, and other than refereeing the mechanical bits, enjoys the show.
