Bob Rock worked with Metallica on The Black Album after producing Mötley Crüe's Dr. Feelgood.
Metallica admired Rock's work on that 1989 album and decided he could help them make the strongest record of their career. His hiring was a deliberate change from the production team associated with much of Metallica's earlier work, especially Flemming Rasmussen, who produced Ride the Lightning, Master of Puppets, and ...And Justice for All.
Rock asked the band to approach recording more like a unified live group. Instead of building many rhythm parts separately, the musicians worked together more often, while Rock also emphasized clearer vocals and a more polished, spacious drum sound.
The collaboration became controversial because the sessions were stressful and involved repeated arguments. Nevertheless, the result was commercially enormous. Rock remained involved with Metallica on later albums and projects, making The Black Album the start of one of the band's most important producer relationships.