"Bite me."
It isn't something to be taken literally, and he would have expected him to know that by now. How many years had they known each other and he'd heard him say that, and not meant it in a literal sense? But even so, there was Cas, raising an eyebrow at him and moving closer.
"If you insist," he says, his tone completely serious. And Dean doesn't really expect him to do it, but there he is, getting in his personal space and suddenly his teeth are on his shoulder.
"GOD DAMN IT CAS! I WASN'T BEING SERIOUS!"