Tom Morello "learned 250 songs" before his first tour with Bruce Springsteen and the E Street Band.
The Rage Against The Machine guitarist - who was part of the 'Born To Run' musician's legendary touring group from 2008 to 2015 - admitted he still felt unprepared for the frontman's habit of taking random requests from the audience.
Responding to a fan asking about the "initiation ritual" for the band, he told Uncut magazine: "The E Street Band was one of the greatest live rock bands in the world for 40 years before I played with them, so my main job was not to ruin a perfectly good thing.
"I learned 250 songs before departing for the first tour, so I guess that would be the induction. And still it wasn't enough.
"Bruce would often surprise us by taking requests from the crowd - and sometimes they weren't even Bruce Springsteen songs. When we were in Australia, we played songs by INXS, the Bee Gees..."
Tom insisted that none of the band members have any idea how the setlist will go, and The Boss would even change the "arrangements" on the fly.
He added: "No one in the band knows what the next song is, ever.
"And the arrangements of the songs are also not carved in stone, so you never know when it might be time for a guitar solo. It really kept me on my toes."
Nowadays, Morello has to keep up with his children, and he admitted his 11-year-old is more into modern hip-hop than rock music.
He said: "As a young person it's imperative that you like music that your parents don't really understand.
"in the same way that KISS and Black Sabbath made my mom raise an eyebrow, I raise that same eyebrow, but I have to push it down.
"My nine-year-old loves classic rock, for him it's Zeppelin and Floyd and AC/DC... but sometimes it's so loud."