The Safeco Field appearance marked only the second time the drummer and Novoselic had performed together in Nirvana's hometown since Kurt Cobain's death in 1994.
MCCartney tells Rolling Stone magazine, "We hatched that plan a couple of weeks ago. Dave was going to come up to Seattle with his family, and I said to him, 'Do you think there's any chance the guys would fancy (a reunion)?' He said, 'Well, let me ask.' And they fancied it. So we thought, 'Let's go and flip 'em out'."
Grohl and Novoselic stepped up onstage to back MCCartney on Cut Me Some Slack - the original song he and the Foo Fighters star recorded for Grohl's Sound City documentary last year (12).
The Nirvana bandmates also joined MCCartney and his band on Beatles classics Get Back and Helter Skelter and Little Richard's Long Tall Sally.
MCCartney admits the big surprise was completely drama free: "When you're playing with guys like that, you realise that they've been to the heights of fame, so they're used to it. They're not gonna freak. And they're hell of musicians, so we learned everything really quickly. That's what's great about playing with guys like that."
"They're powerful. It's a great power to have them on stage with you. I mean, my band's great, but when you augment it with Nirvana, that's greater."
In a new clip from the forthcoming official Kurt Cobain documentary, we discover the birth...