Imagine if Jimmy didn’t die, and he was instead somehow trapped in his own body, with Cas also unable to leave, forcing them both to be vessel-mates for as long as the body exists. Jimmy ends up becoming like a narrator, or Cas’s inner voice.
Like your just watching the show and Sam does something stupid:
Jimmy “oh my goodness your all idiots, this is why U keep dying!”
Jimmy “as the original operator of this face, I feel uncomfortable”
Cas flounders in a social interaction:
Jimmy “dude chill, just ask them how their sandwiches are, or if their having a good day. Don’t tell them their darkest secret, that’s not how you make friends!”
Clair says something self deprecating:
Cas “Jimmy will ALWAYS love you, regardless of what you chose or what you do”
Clair “your just saying that, you don’t know anything!!!”
Cas “no trust me I do, he won’t SHUT UP about how much he loves you, please just accept it, I’m losing my mind”
(But obviously Cas secretly loves it and wishes his Dad was more like Jimmy, which of course triggers Jimmys Dad instincts, and suddenly Jimmy finds himself fathering a millennia old celestial wavelength that is controlling his body)
Luci using Cas’ Vessel “PLEASE SHUT UP”
Jimmy on his tenth Dad Joke in as many seconds “oh, don’t get your feathers ruffled!” *he proceeds to laugh at himself*
Lucifer in TEARS “ want out , whyyy meeee!!!!”
But like Cas and the viewers r the only ones who hear him, and sometimes Cas responds to him out loud and Dean thinks he’s going crazy for a bit.