"If Castiel had not been as incredibly charismatic and as complicated as he turned out to be, we would’ve grown tired of that storyline and ended up killing him and blowing the storyline off, like we have in the past to other storylines that haven’t entirely worked for us. The guy who really deserves the majority of the credit for that is Misha Collins, because he came in and he immediately had such chemistry with Jensen and Jared and was able to hold the screen with them. He was just such the total package in terms of giving a really complicated, interesting, charismatic performance, and you just wanted to know more about this character because of him. Ultimately, it’s through Castiel that we view the angel mythology - he’s the one that introduced it to us, he’s the one who expanded it. Zachariah, Uriel, and Anna fleshed out the mythology, and made it so fun, but the fact is that it all started with Misha’s Castiel. I owe him an amazing debt of gratitude for delivering such a wonderful performance and for being a terrific guy and being so easy to work with."
— Eric Kripke on the angel storyline (Supernatural Companion Guide Season 4)