Producer Bob Rock says Metallica’s 1991 “Black Album” “wasn't a fun, easy record to make” and that he vowed at the time to never work with the band again.

Rock has opened up to Music Radar about how he managed to balance Metallica’s established sound with the band’s desire to transition to “the big, big leagues. A lot of people think that I changed the band. I didn’t. In their heads, they were already changed when I met them.”

Rock says drummer Lars Ulrich wanted the band to groove more, so the focus was shifted to his drums, rather than having the drums follow the guitar, like on previous recordings. Rock also explains James Hetfield’s increased songwriting confidence, inspired by writers such as Bob Dylan, Bob Marley and John Lennon.

“I think he saw that he could write for himself but still touch other people,” Rock says. “It was a struggle for him, but he had a tremendous breakthrough as a writer.”

Despite his vow to not work with the band again (“They felt the same way about me”), Rock returned for three more Metallica studio albums (Load, Reload and St. Anger) and two other projects (S&M and Garage Inc).