Prior to his surprisingly good performance, the 25-year-old Horns actor told Fallon that it became an obsession with memorizing complicated rap tracks. "Initially I got into Eminem and was obsessed with learning all the words to 'Real Slim Shady,'" he explained. "There's also a guy called Plan B in England. I've always had an obsession with memorizing complicated and intricate fast songs. It's a disease."
That's when Radcliffe agreed to perform "one of the trickiest, fastest songs" he knows, "Alphabet Aerobics."