Just before The Avengers
Walking by the recreation room and glancing in, Bruce catches sight of Natasha practicing ballet. He knows from their nightly talks that she practices either when she's upset or kept awake from nightmares of the Red Room.
Judging by the time of day, he assumes the former. He stops to lean against the white doorframe and watch. Steve walks by and after bumping into Bruce, he apologizes (only as Steve can) and stops to watch as well. In only a matter of minutes, a small crowd is formed, watching Natasha awestricken.
The music playing on the small purple radio at the back of the room stops and Natasha bows towards the wall. She turns around, wiping perspiration from her forehead, pales at the thought of all those people watching her, then regains her composure and smiles.
"Join me?" She asks Bruce.
He falters, stuttering, "oh, well, I really just-"
"Come on, don't be afraid. We don't have to do ballet."
Tony, being Tony, shoves Bruce into the room.
Glaring at him, Bruce turns and shuts the door before joining Natasha in the middle of the room.
"I can't dance, Nat."
"Then I'll teach you."
"No, I really can't-"
She cuts him off with a kiss on the lips. "There. Better?" After seeing him nod slowly, she takes his left hand in her right, setting her other hand on his arm.
He catches on and places his right hand on the small of her back.
"Let's start with ballroom dancing."
She begins to count and move his feet with her own.
Little do they know, Tony and the rest are watching them from JARVIS's screen in the room.