Let's address the simian in the room. Yes, in his new biopic Better Man, Robbie Williams is depicted as a walking, talking chimpanzee. And no, the human Robbie never appears. It is all chimp, all the time. Think Bohemian Rhapsody if it was set at a zoo. Or Planet of the Apes with extra Gary Barlow. But it had to be this way, insists Better Man's director Michael Gracey, whose previous film was box-office phenomenon The Greatest Showman.

"I think you feel more for an animal suffering than you do a human," Gracey tells me, with characteristically Australian enthusiasm. "There's nothing glamorous about a monkey doing cocaine. It's actually a bit sad and uncomfortable. You're just like... 'I don't want to see him doing that'."

Just as you quickly get very used to chimp-Robbie in Better Man, you quickly get used to talking this earnestly to Gracey - in grey hoodie and matching beanie - about one of the strangest musical biopics in history. And it is strange - an anarchic, moving, impeccably well-made blockbuster about a self-destructive pop star as brilliant as he is irritating. And who is also a chimpanzee, played, via some Andy Serkis-esque motion capture, by the British actor Jonno Davies (though Williams provides the singing and the sparky, occasionally ruthlessly catty voiceover).

Better Man charts Williams's wholesome childhood in Stoke-on-Trent and his ascent to pop stardom in Take That, through to his unexpectedly massive solo career and personal implosion in a fog of booze and drugs. It all culminates in 2003 when he plays to more than 375,000 fans at Knebworth Stadium. (His ill-fated 2006 rap album Rudebox, then, is sadly overlooked.) Sometimes the film plays like an absurdist grand guignol; sometimes a flick through an old issue of Smash Hits ("I'm Nicole..." a mysterious brunette tells Robbie at one point, "... Nicole Appleton!"). It's great fun - and almost improbably so. There is a hallucinatory fight sequence involving dozens of monkey Robbies. An actor in awful Liam Gallagher drag briefly steals the show, and there's an absolutely sensational musical number set to "Rock DJ" that takes place on London's Regent Street involving motorcycles, buses and hundreds of extras.

But wait a second. Musical biopics endorsed by their subjects are supposed to be terrible, aren't they? They're meant to crush decades of music and drama into two hurried hours! They're meant to win people undeserving Oscars! This one, though, seems to have a lot more on its mind. "I wouldn't have done it without the monkey," Gracey says (in the world of Better Man, monkeys and chimps are interchangeable). "For me, the monkey was the only reason to do it."

Gracey had connected with Williams via his Greatest Showman star Hugh Jackman and would go to chat with him in Los Angeles, bringing his tape recorder to hear him talk about his life. There was no real project in mind - Gracey says he does this a lot with creative people, to see if there's a story there to tell. He was struck by the classic drama in Williams's history, his dysfunctional relationship with his pub-singer father, his mischief-making, his feuds and failings. "And I noticed how often he'd refer to himself as a performing monkey," Gracey adds. "He said it enough times that I was like, ooh, this is how he sees himself."

Michael Gracey

Of course, few were particularly eager to make this version of a Robbie Williams biopic. Gracey had encountered pushback on his films before - he was attached to an early version of the Elton John biopic Rocketman, but financiers balked at funding an occasionally trippy, sex-and-drugs-filled musical by an untested filmmaker. Better Man, though, was an even tougher sell. "And this was absolutely because of the monkey," he explains. "But I'd also come off the back of The Greatest Showman, so I had a lot more cachet - I felt I could take a bigger creative swing, and luckily people did eventually get on board."

Impressively, he received no pushback from the real people depicted in the film - despite many coming off as different shades of terrible. The early Nineties rise of Take That seems like a misery, the group ferried from gay club to gay club in the back of a van, while being sexualised and belittled by their tyrannical manager Nigel Martin-Smith ("a first class c***" per Williams' voiceover, a claim they got away with as it's, legally speaking, just one man's opinion.) Gary Barlow is also - quelle surprise - depicted as a smug bore, the finger-wagging yin to Williams's chaotic yang. But Gracey doesn't quite see it that way.

"I actually feel more for Gary while watching the film," he says. "These are young guys, and Gary [the group's songwriter] is putting all this work in and trying to hold the band together while Robbie's passing out and showing up at performances drunk. You feel the frustration because this guy is a massive f***-up."

Barlow was involved in the film's production, allowing Gracey to use songs he's written and looking over the script in advance of shooting. "Rob was really worried," Gracey remembers. "These guys get on really well at the moment - and I know people go, 'well, do they really?', but they actually do. And I think it's hard for both of them to revisit a time when they were..." Gracey punches his fists together.

"You've also got to understand - everyone was just doing their best," he continues. "There was no rule book. There were no guides. No one in Take That sat around and talked about what was happening to them. They were dealing with so much attention and comments and judgement, and really all of them just wanted to be loved."

Appleton, one-fourth of the grungy Nineties pop group All Saints, is an unexpectedly big presence in the film, too - she and Williams dated for several years, during which Appleton fell pregnant. The film makes clear that she was convinced to have an abortion by someone within All Saints' management, to preserve her fledgling career.

"Rob is super protective of Nicole and insisted that I couldn't tell any of that part of his life unless Nicole was on board," Gracey remembers. Appleton was sent the script and signed off on everything in it. "Obviously it's deeply personal, and she was heavily involved - she'd come to rehearsals and I met with her at multiple stages during production." When she sat down to watch the scene depicting her first meeting with Williams, she cried. "She just said, 'we were such messy kids and I wouldn't change it for the world'."

Because Gracey hadn't necessarily set out to make a Robbie Williams biopic, I'm curious what he thinks the man himself has taken away from the experience. "I think he's just surprised - there was no clear expectation on his end when we started chatting that this would be anything, let alone an actual film," Gracey says. "But now I think he just feels this amazing pride. It's a vulnerable film, but that's where its power comes from."

He also thinks it gets to the heart of what fame does to a person. And, somewhat inevitably, it's because Robbie's a chimpanzee in it.

"When a famous person walks into a room, we can't help but look at them," he says. "It doesn't matter if they're even talking or not, we're just transfixed by that person. And that's why it works so well that he's a monkey. Even in scenes where Robbie's not talking, you find yourself just staring at him - oh my god, it's a monkey! And that? That's what it's like to be famous."