Justin Timberlake had better watch out. Jimmy Fallon got busy with another artist for a rap duet on Tuesday's "The Tonight Show." After Daniel Radcliffe revealed his love of rap music, and in particular songs with dense and intricate lyrics, Fallon called out Blackalicious' "Alphabet Aerobics."

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Former "Harry Potter" star said that this was one of the several he had committed to memory, and suddenly it was deja vu. Just like he does before every "History of Rap" with JT, Fallon pulled out a microphone from behind the chair and invited his guest to join him for an "impromptu" performance.

But Fallon didn't need a microphone for this particular duet. All he did was stand behind Radcliffe holding up a stack of cards indicating the letter Radcliffe was on as he worked his way through the song.

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It was an impressive display made even moreso as the song progressed and the tempo continued to pick up. There was only one noticeable hiccup along the way, but The Roots were quick to adjust in their accompaniment.

Afterward, Radcliffe basked in a well-deserved standing ovation from "The Tonight Show" crowd.

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Daniel Radcliffe stars in "Horns," directed by Alexandre Aja and based on the novel by Joe Hill. The dark fantasy saga is set to hit theaters on Halloween night, Friday, Oct. 31.