It was soon after he'd been reassigned that Anna taught him how to dance.
She'd been watching over humans for a long, long time, and she was his superior, so it was part of her responsibility to ensure that Castiel knew exactly what it was he was protecting and watching the growth of. All angels instinctively knew that humans were important, but Anna told him that until he learned about humanity- what made humans different from angels, he'd never really know why.
So she taught him.
One night on duty, Anna brought them to watch over a party. Parties, she told him, changed as humans did, but there was always dancing, and it was always significant. People danced when they were happy, or sad, or angry- they used it to express the whole litany of human emotion. She showed him the artistic ways that people expressed themselves across cultures, and the way that people danced together if they felt strongly connected.
She showed him a pair of lovers, giggling to each other as they held on.
She showed him a father and daughter, celebrating the girl's coming of age.
She showed him a pair of children, holding hands and swaying this way and that in their innocence.
Carefully, she explained what each emotion was, and what people did to express it. But the most important thing that she explained was that dancing brought people together. It was one of the most beautiful things about their father's creation.
After she'd shown him people dancing, she stretched her grace towards his. Still cloaked, they wandered to the corner of the dance floor, and, graces twined, they mirrored the movement of the humans there with their trueforms.
Anna smiled, her grace flaring with pride, when Castiel started learning the moves that she'd taught him. He didn't really understand it, but he continued to move with her, and soon his own grace fluttered in contentment.
Anna explained to him that it was called fun.
Dancing, having fun, was one of the things that made humans so special. They get to do so many things, all on their own, without being told to, and it was the most remarkable happening. Anna asked Castiel if he understood, if he knew part of the reason humans were so loved by their father, and Castiel touched his grace to his sister's in the affirmative.
They stayed where they were for only a few days, but in the souls of those who danced unknowingly alongside them, Castiel saw so many things. All of them were so beautiful, but always so different, no two quite alike, but all of them sharing the spark of humanity that made them glow warmly in their vessels.
On the last day of their watch of that place, Castiel smiled for the first time.
