His expression tells her she's voiced something she usually keeps rather well-hidden. Her scorn for those who don't bother to look past her face and figure is most often disguised as sarcasm, and it's rare that she actually gives voice to her worry that all she'll ever be is someone's pretty trophy wife.

But the potion takes the edge off so many things, and so she stares at him in return, concentrating until he's in sharp focus.

"Rules," she states, agreeing with his comment about walls. "And duty. Proper ladies this, correct behaviour that."

Narcissa hates the confinement of being a dutiful Pureblood daughter... but Andromeda had to go and be an obvious rebel and left her baby sister to pick up the pieces, and there is absolutely nothing in the world that will make Narcissa cause a further rift in the fabric of her family. So even though she inwardly rails against the familial cage, she still allows herself to be kept within it.

"...thank you, though." Because a lady always acknowledges a compliment.