Instead of doing an album of duets, the band took a novel approach, removing Warren Haynes' vocals from the record and replacing them with an all-star cast of singers like Dr. John, Jim James of My Morning Jacket, Dave Matthews, Steve Winwood and Elvis Costello.

Haynes told Radio.com that the idea for the "bonus disc," which is available with the original version of the LP, came about when he asked Costello for a bit of technical advice.

"I'd written this song called 'Funny Little Tragedy,' which was very different, and is very different, than anything Gov't Mule has ever done," he said. "A little more akin to late '70s, early '80s, the beginning of punk, New Wave � the Attractions, the Clash, Stiff Records. It has a little bit of that vibe. We'd never gone down that path before. We'd sent him an email, and I said did he have any advice to capture the vocal sound."

Costello sent him back a very long email but Hayes said the gist of it was, "We always used a very cheap microphone, so use, like, a $100 microphone.'" Which he did. But from that point forward, he thought it would be nice to have Costello sing it.

"I think in the beginning we were thinking we'd just have him sing a verse," Haynes explained, noting that a similar thing happened with Dr. John and Toots, from Toots and the Maytals. "There were two other songs, 'Scared to Live' that we thought about Toots for, and another called 'Stoop So Low' that we thought about Dr. John for, but we still hadn't made the decision to bring any guests onto the playing field."

Soon enough, though, they came up with the idea of using guests on a bonus version of the album. Ben Harper, who guests on the song, "World Boss" on Shout!, previously appeared on the band's 2000 album Life Before Insanity, but according to Haynes, the two have a relationship that goes back to the mid-'90s and Gov't Mule's H.O.R.D.E. tour.