Merlin was being highly impressed by her, and her words and her ways were making him very much find himself fully in like with her. And this like added to her already alluring beauty, and thus increased his desire.
"I will do my best to make sure that you enjoy this evening." he paused, then grinned as his magic reached out, his eyes dancing with faint golden light. She would feel a light stroke along one leg, a gentle touch, as if of warm fingers, stroking above her ankle on one side.