After a long afternoon of fighting anything and everything evil that the Universe had to offer on this particular day, The Doctor and River returned to the TARDIS in the midst of a heated discussion. The native peoples on Crafalla 5 had no idea what the couple were talking about, but they bade them farewell with many thanks for saving their planet. The two bounded into the TARDIS, energized and arguing.
"Your hallucinogenic lipstick doesn't work on me."
"Oh, you don't think so, sweetie?"
"Of course it doesn't," he replied. "I'm a Time Lord, we are immune to ninety-seven percent of the known hallucinogens in the Universe." He pressed a finger to her nose, then proceeded to dance around the TARDIS console and set the ship off for another place and time.
"Why don't we test that, if you're so sure," River suggested, snaking her way around to the Doctor's front.
He swallowed. "Uh, well... That is, that would involve... Humany-ish things."
River laughed. "Fine then, but I'm right."
"Right about what?"
"Right that you'd be affected by my lipstick. You'd only refuse if you knew it would work."
"That's not true!" He protested loudly, working his jaw. She crossed her arms in expectation. "Fine then," he said finally. "One test!"
River grinned and grabbed his shirt, pulling him into a long kiss full of flailing on his part. When she finally let go, The Doctor stumbled back and righted himself. "There, then. No hallucinations, you see!" He walked around the console once more, landing the time machine and talking half to himself and half to River.
She only smirked and walked back out of the TARDIS to wherever they'd landed. He'd figure out eventually that he was really talking to the stairs.