Dean lay on his back, in his room in the bunker. His eyes flickering between green and black. What he had originally assumed, from previous experience (not first-hand though), was that when he became a Demon he would just be a cloud inhabiting a body. However as this was his body, he was not a cloud. He had learnt he could still leave this body as a cloud of smoke, this was a very good way of annoying Sam.
Another thing he hadn't been aware of when he first opened his eyes to this new life was the tail that curled lazily next to him. Sam couldn't see his tail, nor could he feel it, but Cas could and Dean found this fact to be particularly interesting.
When Dean walked his tail flicked from side to side, the opposite way to the leg that he moved forward. When he stood still it curled around his leg. When he was annoyed it would twist and turn like an unearthed worm. When he was angry it would straighten and twitch. When he felt love it swung wide slow arcs behind him. Excitement caused it to wag, yes wag like a dog (He wasn't very happy about that). Dean got pissed that his tail gave away his emotions, especially since other supernatural beings could see it.
But Dean was compensated, quite fairly he thought, with being able to see Cas's wings. Much like Dean's tail, these wings gave away Cas's emotions.
When Cas walked they would spread slightly with each step, as if to counterbalance the movement. When he stood still they were just as stiff as he was, which wasn't quite as stiff as it once was. When he was annoyed they would fluff up, making them seem like the wings of a baby bird (Dean thought this was cute). When he was angry they would spread as far as the room he stood in permitted. When he felt love they would twitch, as if ready for flight. Excitement caused them to flap clumsily, as if the angel could not control them. Cas seemed so much more emotional with Dean being able to see the wings, often the Demon forgot his human brother couldn't see them.
Sam had thought the starring would stop, with Dean being a demon. However it seemed to be the opposite. The staring between angel and demon just seemed to get worse now Dean fit into the category of supernatural being with Cas. Several times Sam found himself screaming, internally of course (although it was getting very close to being external), at them to "JUST KISS ALREADY!" The tension was ridiculous. Sam had watched the pair as Dean's eyes flickered to just above Cas's shoulder and back, black or otherwise, and Cas's eyes flickered to next to Dean's hip and back again. If he had to guess he would say the two were looking at parts of each other invisible to the human eye, he felt very strange thinking about parts of his brother that he could not see.