Dave Grohl and Sir Paul McCartney during the 12-12-12 concert.

In a recent interview with Los Angeles radio station KROQ FM’s Kevin and Bean, Dave Grohl shared about the story behind one of 2012’s most discussed performances between himself, the surviving members of Nirvana along with Sir Paul McCartney – otherwise known as “Sir-vana” [not really].

They performed their high attack tune “Cut Me Some Slack” during the Sandy super storm 12-12-12 benefit relief concert and then once again during the Grammy Awards but not many know how it all came to be – until now, that is.

The unlikely pairing came about while Grohl was putting together a long list of impressive performers for the sound track to his directorial debut, Sound City, based on the famous Van Nuys recording studio of the same nam.

As Grohl notes, he's jammed with Paul "a bunch of times" (the 2012 and 2009 GRAMMYs, among other times over the years), so when he called up Paul to jam about eight months ago while working on Sound City – the Grohl directed documentary based on the famous Van Nuys studio of the same name – McCartney came right down.

"As we were filming all of these performances, I was just like, 'Hey dude, why don't you come down and jam?'," Grohl said during the interview. "He's like, 'OK.' He comes down and he brings the bass that he's played for 40 years and the Les Paul that there's only four of in the world. And then he brings his guitar made out of a cigar box – it's called a cig-fiddle. And he's like (in a British accent), 'Oh I think I'll play this, why don't you play a bit of drums?' And we just jammed, and we wrote this song in about and hour and recorded it. We walked in, we jammed it. It came out of nowhere – the best songs happen that way. We recorded it live and put a vocal over it and that was it. It was three hours and it was perfect."

When asked specifically on the performance for the Sandy benefit concert came together, Grohl was quick to point that it was all McCartney’s doing.

“He called before the 12-12 thing – maybe a month before – and said, ‘Hey we’re doing this benefit, will you play with me?’” Grohl explained. “I said, ‘Yeah, Foo Fighters are taking a break, but if you wanna play, let me know.’ Then he called a couple days later and said, ‘Hey, why don’t we do that song we recorded with you and Krist and Pat?’”

And it wouldn’t be a complete interview if they didn’t ask about Grohl’s directorial debut responsible for all this even being a possibility.

“The movie is really mostly about the human element of music,” Grohl said. “What it’s like to just get together with a person and write a song in a room in a day. It doesn’t have to be perfect but that’s how the magic happens. When you get four human beings playing with each other, you get the Beatles or the Rolling Stones. That’s where the magic happens.”

Pretty interesting stuff; if you would like to check out the entire interview – which also features collaborator Corey Taylor of Slipknot – you can check that out right here.