Inspired by this gif over at Perpetually Caffeinated on tumblr.

.com/post/5066027819/soooooo-did-anyone-else-catch-that-tongue-curl-lip

Castiel watches Dean struggle with the look, impatience and pique warring far his attention. He wants his powers back now. Being useless, being 'impotent' in the eyes of everyone (Dean) chafes the ego he didn't know he had. If he were at full strength he could teleport inside, he could track the Doctor they are seeking with thought alone. He could smite Eve into the ground, make Dean recant his harsh opinions of his uselessness. Throw him to one side with a whim and hold him there, against the wall of this building where no one would see unless he wanted them to, and he'd force him...

Castiel swallows and wilfully suppresses that thought, wherever it came from. It's the voice that suggested that he lay the lips of his vessel on the demon Meg, that small cry for pleasure, for power and ecstasy.

Dean continues to prod the delicate mechanism. Castiel watches the smooth, wet arch of Dean's tongue curl upwards to slide along the back of his teeth. The voice, denied vocalisations, makes heat in him, spreading as he watches that small, curled appendage.

Dean bites his lip in concentration, teeth pressing small white crescents into the soft flesh, still moist from the passing of his tongue. Castiel's own mouth moves a little without his consent, his brain, submerged in simmering heat, imagines what that scrap of plump flesh would feel like under his own teeth. What that hot tongue would be like in his own mouth.

Dean is bent over before him, jeans snug to the long length of his thighs, showing clearly the distinction between one buttock and the other. Beneath his shirt and jacket the flat of his back would be hard, ridged with the bone, a dip where the spine is, damp with the sweat of effort, beading and sliding down to the divot of his back, to the cleft of him.

Castiel can taste salt on his tongue.

His hands itch with the want to reach around the bent body before him, touch the flesh of his stomach, folded and softened by the position he's arranged himself in. He remembers what Dean said to him when he came into his company earlier. What would it be like, to be inside? To feel the strength of Dean rippling around him, laid out before him as the hunter, the man, was laid before him, sweating and gasping in glorious ecstasy, product of human senses, pheromones, and the animal, mechanical rhythm of Castiel's own exertions, driving their flesh together in a mess of sweat and oil. They would smell of musk, feel like skin, like muscle parting, pressing. Dean would taste of salt and skin and blood.

The lock clicks open and Dean stands, turning to catch the dark eyes of Castiel still straying well below their usual focus on his own.

"Are you coming?" the hunter asks, irritated, though perhaps also a little shaken by the unusual tone to the angel's gaze.

Hunger.

Castiel swallows, eyes skating the line of Dean's belt. A low throb of blood makes his heart thump and his innards feel warm.

He nods, not trusting his voice.

Not knowing what would come out if he allowed himself to speak.