Something in Lily's expression changes. For a moment she looks utterly crestfallen; moving to clasp the hand at her knee with both of hers. She rubs her cheek against his hand, and the affection would be real if this were really Sirius. "I gave you the best parts of me," she murmurs, quiet in the face of that syrupy sweet voice that sounds so wrong. "So you're saying my marriage is ruined for nothing? Because you found someone else?"

She's moving though, slinking toward and over him like she's going to try and prove her point. "I have what he doesn't," she says, but as her lips approach his ear her mouth morphs into a sickle-sharp grin. "Like a knowledge of Sirius Black from childhood, you stupid son of a bitch. Did you really think you could fool me? That a caricature that can't even keep facts straight would work? Some bloody Dark Lord you are, can't even fool a single witch all on her own."

And that's the crux of it, isn't it. Alone. She isn't, but he doesn't know that. She's trying to anger him enough to bring out the snake. Perhaps it will work.