Chloe Webb and Gary Oldman improvised their dialogue in the scene leading up to Spungen's death. They created their dialogue from archival interviews and other materials available to them.
Courtney Love auditioned for the role of Nancy Spungen, but was cast as Nancy's friend Gretchen instead. Alex Cox was impressed by Love's audition and considered casting her as Nancy. But the film's investors insisted on an experienced actor for the role. By way of an apology, 'Alex Cox (I)' then cast Courtney Love as one of the main characters in his next movie Straight to Hell.
In his 2007 autobiography, Guns N' Roses guitarist Slash revealed that the casting director hired all five members of Guns N' Roses as extras for a club scene, having coincidentally scouted them in different locations without their knowledge. He said "all of us showed up to the first day of casting, like 'Hey...what are you doing here!'" However, Slash was the only one in the group to stay the entire 3 day shoot.
Though not mentioned in the film, Sid's one and only solo album, which we hear references to in the movie, was titled "Sid Sings". Also not mentioned is the actual name of the band that we see Sid put together while in New York City. They were called "Vicious White Kids", and though they never officially released an album, a bootleg of one of the very few gigs they actually did play can be found to this day, though the recording is of dubious quality.
The line "He washed his feet too much" from Johnny Rotten was based on the fact that original Sex Pistols bass player Glen Matlock would continuously wash himself on tour whenever they arrived at a hotel. This supposedly being one of the reasons that he was replaced by Vicious. It is also mentioned in The Sex Pistols documentary The Filth and the Fury.