Spoilers: Mean Streets (the short story in the Dresden Files universe) and Lazarus Rising (4.1) for Supernatural. You really don't have to read the Dresden File first.
Disclaimer: None of these belong to me. I make no money off of them. So go buy a Dresden book and make Jim Butcher some money.
Author's Note: Jim Butcher's Uriel is awesome and Supernatural's is not. Castiel is cool though, so in my mind, I'm getting the best of the two worlds.
Castiel sat in the choir loft of St. Mary Church in Chicago, Illinois… Earth. He tried to accept how dull the colors were and how the harsh, unmelodic sounds inflicted pain. He had thought that he was prepared for this mission, but his first attempt of communication with his assigned human had failed mightily. His voice was so unlike anything Dean Winchester had previously experienced, even in hell, that Castiel had unwittingly harmed the newly released. The more insistence Castiel had imbued into his voice and the worse the reaction. He had mistaken in his approach.
Castiel had come to this place, to this church, to watch a professional at work. Uriel was speaking with the wizard. He was inhabiting a human. Perhaps that was necessary for Castiel as well? He sent a quick prayer to the Father asking for a human host. There was no immediate answer; obviously God wanted him to wait for something.
Castiel waited. And watched.
Uriel laughed at the human. And then disappeared.
Castiel leaned forward in awe. Uriel had not been in a human host, it had been a human form, unnoticeable to even an angel eye. He had hidden his angelic voice, eyes, wings and even power to such an extent. Castiel shook his head; he would never be able to maintain such an illusion. He did not have time to learn the technique.
"Shaking your head, Brother Castiel?" Uriel said from behind him (in his angelic voice). "Have you not learned that one rarely hears what they wish when they eavesdrop?"
Castiel stood in respect and submission to the archangel. "Sir?" He was confused with the human terminology.
Uriel smiled slightly. "Why are you here, Brother?"
Castiel ducked his head, ashamed of his inadequate first attempt. "I could not communicate with Dean Winchester."
"I wished to observe one more experienced than I."
"How did you hide your power? Put your voice within the range of human ears?"
"The power is not mine to hold."
Castiel frowned at the archangel. "It is the Father's, of course. I do not understand. You did not release it, you hid it."
Castiel knew many angels preferred not to work with Uriel and now he had experienced the reason. "Brother Uriel, please help me."
Uriel eyed Castiel and finally spoke. "Know your human, Brother. The direct approach is rarely productive. Threats, less so."
Castiel was confused. He heard an answer from the Throne: a human host had been chosen for him. He nodded his farewell to the archangel and flew toward the host, considering Castiel's words.
He did not understand. He did not know if he ever would.