Regulus tilted his head minutely to one side as he observed the brief but perceptible shift of discomfort in Rainer at his compliment. He might be a little clueless but not completely, mind you. He fought the smile that tugged at the edges of his mouth at this observation - figuring it was just because she didn't like him pointing out just how dressed up she was.
Still, her reaction was curious - if she hadn't been his best friend for all those years he doubted he would've picked up on it at all.
Regulus was about to say something to break the silence when she finally spoke up after taking a few sips from her wine glass. He answered her statement with a smirk of his own and opened his mouth to respond with a return quip, when she complimented him in her own way.
His eyebrows raised briefly before he got his carefully sculpted expression back under control. "Merlin, I think the world's going to fucking end tonight or something, between the two of us being so nice to one another." There, he called her out on it without making it too obvious and managed to turn it into a bit of a joke even.
"And see, further proof that I am capable of dressing nicely, should the occasion arise...or when forced," Regulus added, with a flick of his ridiculous bow tie.
For a split second, his gaze had started to drift in a direction that it shouldn't and his eyes quickly darted out onto the rest of the party. Without missing a beat, Regulus spoke once more, "Don't even think this gets you out of the other deal we made. I'm still going to show up on your doorstep in another suit - if only to see the look on your face."