"It's always Harry this and Harry that. I'm hairy. I'm really hairy," Pegg said as the fictional Harry Potter character. "Have you seen Fifty Shades of Grey? I got 50 shades of ginger right here," he said while starting to disrobe.
Before Pegg could get too far, Fallon quickly cut him off.
"Ron, no one wants to see your magic wand," Fallon said.
"What about my chamber of secrets?" Pegg asked, turning around to show his backside.
Pegg was on the show promoting Mission: Impossible - Rogue Nation in which he stars opposite Tom Cruise. The action star notoriously does his own stunts in the movie, including the car chase scenes that called for Pegg to ride shotgun. The experience was draining for Pegg, but not for the reasons you would think.
"He turned your seat heater on," Fallon said.
"Yeah, hilarious," Pegg answered. "We're in Morocco in very hot weather, and I'm sitting there like, 'It's really hot in this car, Tom.'"
Pegg revealed that this prank lasted for two days until he caught Cruise in the act and decided to turn the tables on the Golden Globe winner.
"The end game was marvelous," Pegg said. "[Tom] went to the bathroom and I called over one of the camera guys and said, 'Give me a really small sliver of black tape.' I put the tape over the light of his seat heater so he couldn't see that it was on, and then I put it on full blast. He came in and sat down and cooked."