At least I know what I'm trying to accomplish, sort of:
Basically, for this one scene, my comedy of manners (with special bonus sexy bits!) takes a detour into the realm of fairytale. Our heroine, Lucy, has a new dress for her first ball, and it's a transformation. Until now, most of the people around her have been perceiving her as and treating her more like a 14-year-old than an 18-year-old--partly because the fond aunt who brought her up has been unwilling to let her youngest baby grow up, and partly because she's the petite one in a family of tall, curvy women, and they subconsciously have trouble realizing that a cousin/niece who's half a foot shorter and an order of magnitude flatter chested than any of them is fully grown.
In reality, Lucy is very mature for her age in almost all ways--responsible, level-headed, and clear-sighted. She is, however, quite innocent (though not ignorant) and not quite sexually awakened. So first, she needs to see herself in a new light. She's got this new dress, which she loves like any girl should love her first ball dress, and she's happy that the way it's cut makes it quite obvious that she's no child. But she's also a little frightened of herself. The dress (which I had a lot of fun imagining) is of purest virginal white, but low cut, and trimmed with a few elegant touches of crimson. Her maid has braided crimson ribbons into her dark brown hair, and replaced the gold chain on which she usually wears her amber cross (the sole ornament she owns) with a thin matching ribbon around her throat. Lucy instinctively feels, though she wouldn't have the vocabulary to describe it, that there's something very erotic about this snow-white and blood-red combination, and she's not sure what to think of herself.
So, she then needs to make her grand entrance. She's running late, so the rest of the family is already assembled at the bottom of the stairs. (The ball is at the castle where they are staying as guests, because her eldest girl cousin is engaged to its owner.) Everyone, except for clueless-but-mostly-nice boy cousin Julius, needs to immediately notice how different she looks.
Immediately thereafter, our hero (James) enters, since he and his sister are among the privileged guests invited to dinner before the ball. He hasn't made the mistake of thinking of Lucy, whom he has only recently met, as a child, but he also hasn't been attracted to her--he's impressed with her good sense and grace under pressure, which he's already had ample opportunity to observe, but if you asked him what she looked like, he'd just frown and say something about fine eyes, but he hadn't really paid that much attention. So when he sees the new Lucy, he needs to be gobsmacked, and she needs to notice. A nice little moment of breathless awareness.
If I can get through that, I think I can manage the rest.