Something Castiel never got bored of:
breathing.
He didn't know if he had to do it, if he had to keep the heart drumming, the blood churning, the eyes blinking wet. What he always kept doing, though, was what Jimmy had always done. He kept breathing. And it never got boring.
Sometimes he'd do it on autopilot, and then he'd remember and listen to the air suck in and out of him like a tide and feel the grate of the oxygen on the back of his throat and he'd swallow down excitement. Tastebuds were like that, too. Tasting was human. The smooth gritty tongue surface that responded to cheese and onions and meat and bread and the nose that kept on breathing and savouring the air and breathing.
It was different, it was so different, to sustain a body, to bite down with borrowed teeth and lick the crumbs off the lips and breathe in, breathe out.
Ghost itches, the shadow brush of eyelashes and the minuscule hairs on the face and the arms and the thicker ones all over the head. Every smell and taste, constant input, constant sensation. Humans know how to block it out. Castiel learned after a while, but sometimes it still hit him and he sat there, hearing thinking smelling feeling everything, re-learning the instincts he needed.
He could feel the bones nestled in his skin and the brain throbbing in his skull and the toes bunched up at the end of his feet. It overwhelmed him. It was like a miracle. His father's miracle. Concentrating was an island of solace in the middle of confusion, doorways to doubt, mistakes and paths walked wrong. To just massage the joints in his fingers and gently pop the vertebrae in his neck, feel the wetness of his eyes and the pounding of his small, blood-filled, desperate heart.