It felt a little like being interrupted by a grown-up, which was absolutely ridiculous because Lucius Malfoy was barely a handful of years older than they were; Lily was already shoving down that kneejerk instinct, pulling up a pleasant smile, when suddenly there was a hand, and an invitation to dance. "Um," Lily said, before she could really stop herself, and that wouldn't do at all. But she couldn't say no, not without good reason, and she didn't want to not dance at all, so that only left...
Offering a smile of her own. Setting her hand in Malfoy's. Saying, "Why not? It sounds delightful."
And shooting a pointed glance back at James to not say or do anything stupid.