"Is it really? Could you really do it?" he asked, raising an eyebrow in disbelief. He didn't believe the Master would ever kill him; they were best enemies, what would they be without each other?
He paused for a moment as the Master spoke of traveling together. As much as the Doctor wanted to believe it was true, he wasn't quite as naive as the Master might have thought, despite his weaknesses. "Uh huh, and what's brought this on, then?" he asked, a little exasperated.