Bob Rock produced Metallica’s 1991 album and later played bass on St. Anger. His production credit on The Black Album came more than a decade before his temporary performance role on the 2003 record.
Rock became closely associated with Metallica after the band chose him to produce The Black Album. His work helped shape its controlled, massive sound, which contrasted with the highly intricate approach of ...And Justice for All.
When bassist Jason Newsted left Metallica before the recording of St. Anger, the band had not yet officially recruited Robert Trujillo. Rock performed the bass parts for that album while continuing as producer. Trujillo joined Metallica after St. Anger was completed.
This distinction often causes confusion: Rock was not Metallica’s permanent bassist, and he did not replace Newsted as a long-term member. His bass performance was specific to the St. Anger sessions, whereas his production relationship with the band began with The Black Album.