He taught 'School of Rock' but Jack Black is now learning the art of bluegrass. The comedic actor lends his vocals to 'Old Joe Clark,' a song on his father-in-law Charlie Haden's Grammy-nominated bluegrass CD, 'Rambling Boy.'

"I wasn't sure what to expect, because I haven't recorded or really sung any old songs like that before, bluegrass style. But it came very naturally, and I cranked it out in two takes," Black tells the Associated Press. "There was something in the music that I think struck a chord in my DNA. I think I've got some hillbilly in my roots."

Haden intended 'Old Joe Clark' to be an instrumental, but Black talked him into adding lyrics. Black's wife, Tanya, is also heard on the album singing with her two sisters on 'Single Girl, Married Girl' -- an old Carter Family favorite.

Black is a musician in his own right, fronting the rock band Tenacious D. But will he cross genres now that he's found his inner 'hillbilly'?

"I'm already practicing my square dancing if we play the Grand Ole Opry," he jokes.