The Master denies his request--or order, really, it was more of an order than a request, the Doctor has to admit--and for a moment, the Doctor's certainty wavers. What if the Master means it? What if he really doesn't want this?

The smile on the Master's face says different, though. The Doctor knows that smile; he's being tested. He answers with a slow smile of his own, lowering his hands to his trousers, undoing buckle and button and zipper and slowly undressing, tossing aside his jeans and boxers to join his shirt on the floor.

Then he just stands there before the Master, running his hand idly over his cock, waiting. He believes he told you to lie down.