Grohl came by the part pretty naturally, as he'd already worked with Black and Gass. The duo had been touring their act, Tenacious D, since circa 2000 as live shows, but eventually the comedy-musical act turned real band, complete with recording a couple of records with Grohl on drums and a brief "Tenacious D," series on HBO, per Rolling Stone.
According to Foo Archive, Grohl's friend and former bandmate was working at The Viper Room in L.A. where Tenacious D was regularly playing on the weekends. He told Grohl he should come check out the act, Grohl did, and from then on, Grohl, Black, and Gass have been buddies and have done various projects together.
As a testament to their friendship and Grohl's appreciation for Tenacious D, he told Black in a conversation published on Foo Archive, "I've always said that I'd never do a movie and that's a sacrifice that I've made for you time and time again. But I got to play Satan in the Tenacious D movie! That's like Kurtz in 'Apocalypse Now!' You can't beat that!"
Though, Grohl did say the make-up was brutal — several hours of sitting there as they put it all on. "But it was great because the whole time I sat in the chair and drank Jager and listened to Slayer, Unleashed, Mercyful Fate ..., Grohl said. "I just listened to metal for six hours so when it was like, 'Mr. Grohl, you're on' — I was just like the devil!"
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