Cas sitting and reading a book in the bunker’s library - not for business, but for pleasure. He’s lost in the story.
Dean coming in and walking up behind Cas quietly, meaning to surprise him. Cas doesn’t notice, his focus on the story too complete for anything to bother him.
Dean reaching out his hands, meaning to pinch Cas’ shoulders suddenly, and make a loud noise at the same time - just to give him a bit of a shock, see what happens when an angel jumps.
Dean’s hands softening at the last moment - and before he knows what’s happening, the pinch becomes a gentle squeeze. It’s careful and caring, both. He’s the one who’s shocked - at himself, at this impulse to be affectionate. And Cas is turning around to look at him with wide, happy eyes.
“Good book?” Dean says, to cover the moment. His hands haven’t moved. Cas reaches up, and presses his palm to the back of one of them.
“Very good,” he says. “You should read it some time.”
Dean awkwardly patting Cas’ shoulder and heading off on his way.
His hands tingle for the rest of the day.