Castiel likes to watch Dean, especially when he's alone.
When Dean is alone and Sam can't be expected back in the room for a while Castiel likes to just sit and watch, invisible, in silence. Sometimes Dean doesn't really do anything. He'll sit, watch TV. He'll think of the things ahead and the obstacles yet to come. Sometimes he reads, though he would never let Sam know. He likes Lord of the Flies. Sometime she feels like Ralph. He even Jack at times, like after a particularly fierce hunt that ends bloody.
Rarely, though it does happen, he feels like Simon. He feels like he can relate to the boy upon his death. Being torn apart by those he once saw as people who were safe, people who were friends. That reminds him of his brother and Ruby and Demon blood so he usually tries to stay away from that train of thought.
What Castiel likes to watch the most, though, is when Dean will undress himself until he is either naked or laying in nothing but his underwear on his bed. He'll snag Sam's laptop and start playing music. Of course he plays music often, clothed or not. But during the times when he strips himself of all his layers of clothes, bares himself to the seemingly empty room, he always starts with one particular song.
Kashmir, Castiel has learned that it is called, by Led Zeppelin.
He'll play the music as loud as the laptop's speakers allow and he'll lay down, close his eyes and just listens. The angel sometimes wonders if he is at all aware of his body's reactions to the music.
His heart hammers at some parts. His legs and arms and fingers twitch. Occasionally he'll arch his back just slightly. It's like he wants to throw himself from the bed and just run until his body gives out on him and he falls to the ground. But the music has him pinned to the mattress.
Castiel wonders if the feeling his smothering, like he's trapped. Or if Dean just feels free. He wants to know what his human is imagining in that head of his but he promised he wouldn't invade the other's mind. So he doesn't get to find out what imagery is playing out on the backs of Dean's eye lids. He just watches, quietly, and listens to the man's music.