His gaze slides slowly from the younger Narcissa to Lucius, so carefully without comment that it pretty much has to be a comment in and of itself. If this were a more... 'normal' situation, he might just drift over to knife-wielding Narcissa's side of the kitchen and stay there. Because knife-wielding Narcissa is less alarming than whatever's going on with Narcissa and Lucius.

However, he's not even acquainted with this Lucius. At least he's talked to the other Narcissa before or would have if his typist hadn't flaked out on that. A more socially adroit person might strike up a conversation, suggest some form of diversion like chess, or anything, really, apart from stare curiously at Lucius. Which is actually what is going on.