From the S2 Companion book:
Jeffrey Dean Morgan feels it's all worth it. "I think it's important that the public realizes how lucky we are on a show like that, to get along so well with your cast and crew. There's a lot of laughing...even the scene at the beginning of the season where I die; it's a very emotional scene, but the camera cuts and we're giggling. I can't tell you the dirty things I was whispering in Jensen's ear to make him laugh, but we had so much fun."
Haha, now I really want to know what he was saying...