"You flatter me far too much," Moriarty croons at the distrust of her handling basic objects, finds it amusing that everyone's too suspicious to give her even the smallest allowance. Entirely justified as it is. The more they deny her, the more empowering it is, feeds her ego by being too dangerous for them to handle without the highest precautions.

She's not going to promise full cooperation- only wants to be as helpful enough to require Joan to keep coming back for more, but not unhelpful enough that it's no longer worth the trouble. "An exchange of information, isn't that what you want? The more details you provide to me, the more I'll offer in exchange," she negotiates the terms calmly, doesn't want to show just how eager she is for involvement. "I was rather pleased with your work on the previous case I had you pursue." She says you to avoid saying Sherlock's name still, and it's probably more revealing.