The first time it happened, Dean was so shocked he just stared.
"You okay, Cas?" That's all he had said. He'd expected a subdued but tragic shake of the head or something equally as characteristic of the angel, but instead he got what started as a yelling match and what ended with Cas screaming at the sky.
"YOU SAID YOU LOVED ME! YOU SAID WE WERE SAFE! YOU LIED! YOU MADE ME DO THIS!" He didn't stop for what seemed like hours, and then he just cried. Dean was so frightened by this change in behavior he didn't know what to do. He wanted to kiss it away, to kill whoever had done it, but that was the problem. He was already gone.
That night was the first night, but it happened at least once a week. Dean was wary or it, all the time. Cas would drink almost all week, and then one day he would just start crying. Dean would hold him, rocking him back and forth as the other man soaked his shirt shoulder, and he would kiss his forhead and lie to him about how it was going to get better.
Everything was new and different and hard. An angel had never been cast out like this. It was one thing to fall, but that's not what had happened. He hadn't fallen, he had just been silenced. And he felt like humans did, all the guilt and shame and regret. How did they stand it?
