Everyone has different deep down fears and Supernatural is showing that in different ways.
Deans and Sams are not the same but they're simmilar: the fear of hell.
For Cas it's also obvious: kill Dean - his love - over and over again. Even as angel with autistic behaviors he hast to fight his Nightmare over and over again.
Charlie? Her fear is being alone, having no family, being bullied and excluded, and never being truly seen for who she is and for how she behaves. Her fears are so normal and could be from nearly everyone else. But it is soo well played and written that we can feel with her.
Chucks fear is to be meaningless and gets outsorted from humans. He wants his power and showes it regularly. But he fears also beeing lonely, have no fans, no one who believes in him and left alone from all chatacters and his family.
Crowleys fear is also his addiction in my Point of View. He doesn't want that any demon sees him beeing addicted to be human and/or to pure human blood and his feelings coming back. But his fear is also be outsorted and be not necessary anymore. He loves his position in hell even when he doesn't liked hell.
And at the end I've got Rowenas Nightmare: Fergus in his pyjama beeing happy about a funko-pop-Sam at Christmas. I love that soo much because who would think that Fergus loves Sam, celebrate Christmas AND acts all in all like a child?
But Rowena fears even more Lucifer and his tortue and beeing unseen, unheard and beeing underestimated. Her fears are soo good seen in her actions when shes scared and cornered.
