Mick and the backup singer
In the book, Mick talks about how he and one of the backup singers became involved, and the guys, who of course went through women like Kleenex, suddenly became moral and disapproving. "Hey man, no fraternizing! She's gonna break up the band, man!" I think there was an allusion to this: in a late concert scene, the same woman singer is shown twice, after which Mick looks to one side, presumably at her. I guess that was an allusion, but if I hadn't read the book, I wouldn't have known.
Overall, this had good economy of story. But I wish they'd had a line of dialogue supporting this, or a shot of Mick and Singer together, with the other three glaring at them.