Grohl detailed this unexpected collision of hard-rock and mainstream pop in Rolling Stone's new, in-depth cover profile, noting that Timberlake often hung out with the band during downtime between their respective recording sessions at L.A.'s EastWest studios.
"We'd drink whiskey in the parking lot," said the Foo Fighters frontman. "He was really, really cool. Then the night before his last day, he says, 'Can I sing on your record? I don't want to push it, but – I just want to be able to tell my friends.'"
Timberlake's contribution amounted to "la la la" vocals on one track, but Grohl was adamant that his new friend "nailed" the part. "I'm telling you, the guy's going somewhere," he joked.
Grohl first teased the collaboration in June, mysteriously telling BBC Radio 1 that the world's "biggest pop star" sang backup on one of the album's "heaviest songs." The following month, the guitarist walked back that description a bit, explaining to Rolling Stone, "This person, I think, more than [just a pop star]."
"I'd be amazed if anyone can really figure it out," he added. "But it was great and this person's been around a long time. And I think I've met this person maybe a few times, but I'd never spent time with this person, and we had a blast together. Really fucking fun. And [he or she is] very talented – more so than I ever knew. But, of course, I fucking ran my mouth off; now I have to answer to it every time I pick up the telephone and I'm like, 'Fuck! I shouldn't have said anything.'"
Timberlake isn't the only major cameo on Concrete and Gold: Paul McCartney plays drums on one track. "Even if it had been banjo, I think I probably would have showed up," the former Beatle told Rolling Stone of his jovial session.
Concrete and Gold, out September 15th, also includes guest spots from the Bird and the Bee's Inara George, the Kills' Alison Mosshart and Boyz II Men singer Shawn Stockman, among others.