Metallica hired Bob Rock after admiring his production work on Mötley Crüe’s Dr. Feelgood.
Released in 1989, Dr. Feelgood was a major commercial success and demonstrated Rock’s ability to give hard rock a powerful, clean, radio-ready sound. Metallica wanted him to bring some of that impact to their next album, even though the band initially hesitated about allowing an outside producer to guide the sessions.
Rock’s methods differed from the approach used on Metallica’s earlier records. He encouraged the musicians to play together, sought a stronger live feel, and pressed James Hetfield to improve the lyrics. He also asked the band to record many takes, sometimes reviewing roughly 20 to 30 performances before choosing and editing the strongest material.
The collaboration changed Metallica’s presentation without removing its heaviness. The resulting album became slower and more spacious than the band’s earlier thrash records, a shift that helped it reach a far larger audience. Rock later worked with Metallica on several additional albums.