On the same show a few months ago the Harry Potter star admitted he had a penchent for memorising difficult hip-hop lyrics, and that he knew all the words to 'Alphabet Aerobics'.
Fallon's in-house band The Roots immediately challenged him to prove it by performing the track on air. Radcliffe initially refused, but when he returned this week, he decided to give it a go.
Watch Radcliffe give 'Alphabet Aerobics' a go below:
At a press conference promoting his new film, Horns, Radcliffe said: "The producers said 'The Roots really want to do it with you', so I was like, 'Ok!' But after I did the rehearsal and Questlove (The Roots' drummer) came back into the dressing room, he said, 'Yeah man, I'm gonna text those guys (Blackalicious). They're gonna be really excited. I produced their second album'. "I was like, 'Oh, Ok, you are friends with these guys!'. So I might not have been as bold to suggest it (the rap) had I known that. But I'm amazingly glad I did it. I was shaking with adrenaline a couple of minutes after; it was awesome."
Last month Radcliffe spoke to Gigwise earlier this month about his other musical loves: Pixies and Pete Doherty. Read the interview here.