There’s a black limousine creeping along Sunset Boulevard late in the evening of 16 September 1991. It stops short of Tower Records, which looks like Dracula’s castle under siege, with a rowdy mob of people drinking, swearing and banging on the locked doors. Although he’s risking a full-scale riot by doing so, a dark figure slips from the limo, sneaks unnoticed into the store’s trade entrance and takes up position behind the same two-way mirror through which he was spotted shoplifting cassettes as a teenager. It’s voyeuristic of Slash, but you’d do the same.

At the stroke of midnight, 4.2 million copies of Use Your Illusion I and II will be released to the US public, marking the largest album shipment in history. The releases also draw a line under a period that dragged Guns N’ Roses through hard drugs, firings, food fights, public nudity and – let’s not forget – some of the best guitar riffs of the decade. “I’d have preferred to do a record with just 10 fucking songs that were a bit more straightforward.” Slash

Looking down on the ringing tills, Slash doesn’t know it yet, but these records will be the last great statements his band makes before it fractures, falls and mutates into the freakshow that trades under the Guns N’ Roses banner circa 2011. That’s all to come. For now, everyone is listening to Use Your Illusion. Everyone, that is, except Slash himself.  “I don’t know which one I prefer,” he tells us straight out of the blocks. “I haven’t listened to the Use Your Illusion albums for so long, I don’t even know what’s on each. I know people like the blue one over the red one… Or maybe it’s the other way around.” Er, right. It’s a worrying start to an interview that’s based around the 20th anniversary of these twin albums, but things soon warm up. It’s not that Slash doesn’t care. It’s that while making the Illusion records, he cared about nothing else, losing himself to these 30 songs while holed up in Studio B of LA’s Record Plant as a kind of heroin withdrawal programme.

There are memories in this music – musical, personal, glorious and painful recollections– and you sense that listening to it is like sifting through the old photos from a failed marriage. “I don’t like to go back and look at stuff, because I find it mesmerising,” Slash says. “It freaks me out. So I avoid it.” Push him on it, though, and the dam breaks: “I was just totally obsessed with the creation of the Illusion records and when I got into that studio, I was completely absorbed with everything to do with them, all the time. Because it had been so long.

“We’d made Appetite [For Destruction] and then toured for years and – for me, and I know for a couple of the other guys – we’d crashed and burned. So we were pulling ourselves out of the fucking quagmire and going back to work.” How bad did things get pre-Illusion? Try rock bottom. An on-and-off heroin user since the late ’80s, Slash cleaned up for Guns’ first serious stab at the albums: a residential preproduction session in June 1989 that saw the band and crew relocate to Chicago. If this was intended as a team-building exercise, it tanked; Guns’ troubled second guitarist, Izzy Stradlin, often failed to show and Axl Rose drifted by sporadically to jam at the piano. While kicking their heels, Slash and Duff McKagan managed to combine a daily half-gallon of Stolichnaya vodka with an unlikely interest in weightlifting. This wasn’t going well.

Still, Chicago wasn’t a total flop. The seeds of a few songs emerged, notably Bad Apples, Garden Of Eden and Estranged, with frontman and leader of the band W.Axl Rose pounding the rehearsal-room piano and Slash wringing rich vibrato from long, hanging notes. But it was becoming clear this new project’s grandiose ambition was a sticking point. Making an album to soundtrack fighting and fucking was no longer enough for Guns’ lead singer. “We want to define ourselves,” Rose told Rolling Stone. “Appetite was our cornerstone, a place to start. That was like ‘Here’s our land and we just put a stake in the ground. Now we’re going to build something.’” Slash had misgivings, but didn’t want to start rocking an increasingly precarious boat. “It was definitely exploratory compared to Appetite,” he explains. “I mean, honestly, I’d have preferred to do a record with just 10 fucking songs that were a bit more straightforward, but it was an opportunity to finally get the band to work again.”

According to his autobiography, we remind him, Axl was starting to communicate with the band through management. “Me and Axl were doing okay,” he sighs, diplomatically. “The only catch with the Illusion records was the introduction of synthesizers. I disagreed with synthesizers – and I still do. By the time the Chicago sessions collapsed, the band had become boorish and bad tempered, with Axl dumping the band’s Italian buffet on hecklers beneath their apartment and ejecting groupies for failing to deliver.