Only Lemmy and Wino visited Studio 606 to record, with all the others sending tapes from studio to studio until the album was finished. Soundgarden guitarist Kim Thayil plays additional guitar on two tracks thanks to Kasper, who had brought the Probot tapes to Seattle and they attracted Thayil's interest. Grohl was pleased with the addition as Thayil had more experience with lead guitars, while Grohl was "more about the riff and the rhythm". A bonus track titled "I Am The Warlock" was provided by comedian/actor/musician Jack Black. According to Black, who described the song as "a homage to 'Iron Man.'" after Grohl approached him regarding writing lyrics, his spouse Tanya Haden suggested "it should be about a fucked up relationship," so Black made it about a warlock.