The Doctor watched Jack step away impassively, making a show of not responding. All show, of course. He knew Jack was loving this, and there was no way he'd stop. The way he smiled before, the gleam in his eye, the Doctor was fairly certain that even his colder former self would've been fascinated.

"Nothing," the Doctor said, firmly. "I don't want you to do anything."

He couldn't stop himself from smirking. "Because, you see---" he took a step towards Jack. "---you're the control. The variable affects the control."