Charlie Harper was a sinner and he was aware of that, enjoyed it even. Since he had the idea that he would not qualify for heaven he should enjoy his life as much as he could.Surprisingly it didn't take much for Castiel to get him to agree to accept the burden and privilege of becoming an angels vessel. Just a simple question and Castiel took his new body and did gods work. And if the human had been drunk at the time, as vessel he was guaranteed a trip to heaven. Castiel decided early not to visit Charlies heaven.
Castiel had always thought that humans were mysterious and complicated beings but he never thought acquiring a vessel was that hard. He had explained as much of the process to his vessel as was understandable to a human but Sheldon Cooper continued to resist.The human contradicted him on the most basic principles and continued questions. The answer that the string theory was as wrong as the loop quantum theory had angered the scientist to a degree Castiel couldn't understand. The angel basked in the knowledge that even if Sheldon resisted he still had Missy Cooper as a possible vessel.
Castiel watched the actions of his future vessel carefully. It was so interesting what humans were willing to endure, what they chose to endure without any need. What amused them.Grabbing this opportunity the angel knew he would have a lot of questions. Lady Heather obviously enjoyed her work and knew what she was doing and how far she could go. She was intelligent, well-spoken, had a wide variety of knowledge and more insights in the human psyche than many others. She was the perfect vessel. Castiel looked forward to this. Having her as his vessel would be most educational.
Castiel concentrated his energy on his possible vessel. Castiel saw immediately that his very essence was wrong. Jake Gray was a demonic hybrid. Not a cambion, the demonic looking species he descended from was a lot rarer then the usual demons. It was surprising that Jake had never done anything Castiel would have expected given his heritage, his half-human soul was innocent. The angel knew he would never use him as a vessel but the fact that Jake overcame his nature and lived in a secured cell gave Castiel the knowledge that he wouldn't harm anyone. He let him live.
The first meeting with his vessel made Castiel clear that the human was already occupied. The body of Tom Haniger was a rare case of a ghost possessing a human. As an angel it was easy for Castiel to burn the squealing and squirming non-corporal out of the human host, leaving it free from supernatural influences so he himself could take control of it later. First he soothed the frightened soul that was finally free and engulfed him in the equivalent of a careful hug. The angel enjoyed the comfortable feel of the finally relaxing soul against his own wavelengths.
His vessel was interesting. He had known of these transgenics, genetically modified humans but he didn't expect his vessel to be one of them. His first contact with Ben had gone surprisingly well, the young X5 had a deep faith that was rare in this time and day. Castiel had to admit that he was pleased with these news. Not only made it his work easier, it made clear for him why Father had decided to accept these variations of divine will: Despite them being created in a lab they still had the free will and beautiful souls of humanity.
Castiel never thought about the way vessels or humans in generals looked to the reduced abilities of the few human senses. Priestly's need for constantly changing his appearance was just an oddity the angel filed away as part of this specific humans personality, the same way he preferred to his given name. Coming from a deeply religious background before Priestly had decided to live on his own made it surprisingly more difficult for Castiel. The knowledge that Castiel was an angel had not been enough for young man, the information that he would help his fellow humans changed that easily.
Edward Morgan Blake was not a good person. Castiel knew that the human was aware of that and he didn't make any excuses for it or his behavior. As much as it pained the angel to see so much disregard for human life, the Comedian as he was better known was surprisingly insightful when it came to others. Even with his love for humanity and everything it stood for, Castiel shared Edwards looks from the outside. The same thing that had made this ruthless human the way he was, gave the angel a unique perspective on both humanity and heaven.
Castiel had been aware that humans were not only about choice and free will but also about variation. He never expected his vessel to be so... crazy, manipulative and headstrong but despite all her ticks Lois cared for her family and wanted the best for them. Castiel wondered for a moment if her loved ones would be able to cope with losing her dominating personality to keep them all in check. Or more specifically, if he himself would be able to suppress her completely to finish his assignment. He was willing to risk it. If nothing else, she was interesting.
Castiel knew that humans were able to have more than just one set of personality but Bruce Banner was an extreme case of duality. In his human form he was a soft spoken, gentle man, driven by his search for inner peace and knowledge. In his mutated form his only desire was destruction. Sharing his body with an angel would give both sides of his nature much needed rest. The angel decided that trying to force both personalities together like other humans would damage his vessel, make him less than he was now. He was not surprised by Banners acceptance.
Castiel wasn't sure what to think of Iwan Iwanowitsch Goratschin. Not the mutation, neither the para ability in itself nor the unusual look of the body. No, the strange part for the angel was to be confronted with 2 identities and personalities in just one vessel. He assured the mutant that he meant no harm, that he was on earth for good reasons, to guide the person meant to save his race. Both brothers decided to give him consent for their body. Castiel wasn't certain that it would have been required in this unusual case but he preferred it anyway.
Notes: I expect most people not to know him. If you want to picture him, he is a siamese twin (one body completely shared, two heads, both identities good natured), 2,5 m hight, green scaly skin, with the mutant ability to light fires on the atomar level
Castiel was a warrior. It was the way he had been created. There was a certain irony to it that his vessel would be a natural healer, practically his opposite. Getting to know Leonard McCoy taught him that it wasn't that easy, that despite his need to heal and help people and his deeply pacifistic attitude he was still able to do the part that was requested of him. He also saw the multiple ways his mind had been tampered with and harmed. No, Castiel would not ask that of him, there were still other possibilities in the McCoy line.
It was deeply satisfying to the angel to find his vessel to be both a genuinely good person and a believer at the same time. Abigail Sciuto embodied all the good aspects of humanity, the kind nature, the strive and yearning for knowledge, the willingness to do her part to make the life's around her better any way she could. She was a good representation of what their father had envisioned. Castiel was looking forward to experiencing the world and species that had produced such a remarkable human. Her fast agreement to be his vessel didn't come as a surprise.
Jacob Glaser was a lot of things, intelligent, sharp and deeply paranoid. It would take a lot of work to assure the human that he was an actual angel. While the man believed a lot things his colleagues dismissed outright, religious teachings were not a part of that. Surely Castiel had to be an alien. But as far as the angel knew the species who was most likely to get to the human world in the neighborhood, universally speaking, didn't master faster then light flight yet. The angel also didn't understand Jacobs fascination with Loki's trick with the robot head.
Castiel wasn't sure what to think of his potential vessel Dean Forester. He was a human who had made mistakes but was essentially a good guy. He had made peace with his former wife and his family, trying to make better choices. The angel took notice of his good intentions and the personal desire to be a better person. Yes, he would be able to work with that. The human would benefit from the knowledge that his single slip of unfaithfulness wasn't the end. He would be an adequate vessel. Convincing him to serve as Castiels vessel would be easy.
The vessel was damaged. It didn't take Castiel more than a short moment to figure out that Denny Duquette wouldn't be able to live for long without angelic interference. Not only would dying or terminal ill people easier accept their identity and role of angelic vessel according to heavenly knowledge, their souls were also more at rest, having already come to terms with their separation from their loved ones. It was a much more mutual beneficial agreement than most others concerning vessels. Castiel watched Izzie Stevens cut the line. Denny had agreed, Castiel was willing to wait for his death.
Castiel waited for the abomination to leave before he came. The human was dying but still staying conscious. The angel explained his mission and the human told him his last wish. Castiel didn't need time to decide, he wanted to destroy the abomination anyway, leaving the human aware in his mind for this wouldn't be a problem. The image of Severus Snape covered in blood, going through the fight impervious to curses and hexes, just holding his hand to Voldemort's head to destroy him before Snape vanished into thin air was not something the survivors of the battle ever forgot.