We're surrounded by the utilitarian hurdy-gurdy-ness of a film distributor's office in London, where Jane Pollard and Ian Forsyth, the directors of 20,000 Days On Earth, a sort of Nick Cave Odyssey, have come to talk shop.

Having been friends with the Australian-cum Brightonian musician for years, and having worked with him on previous projects, Pollard and Forsyth had great access. And it shows in their hypnotic and strange film: half-scripted, Brighton nightscape; half stream-of-conscious confessional.

We see the things we expect to see of Nick Cave: in the studio, in performance, in a giant black car. But we also see things we don't expect: snatches of friendship, family life and indulging in pure fancy. Cave is the kind of musician who has a loyal fan base, gives prickly, bemused but incredibly forthcoming interviews, and his renegade-preacher-in-full-sway performances put his rock brethren to shame.

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Pollard, however, rejects any notion that 20,000 Days On Earth is anything other than a film. "We made the film we wanted to make, and he was behind us all the way. We never set out to make a documentary."

They explained that the scenes were planned beforehand, but the content, the language within, was up to whoever was on camera.

The directors often excluded Cave from any discussion of dialogue, but briefed the others (former Bad Seed Blixa Bargeld, duet-mate Kylie Minogue, soul-brother Warren Ellis, blokey pal Ray Winstone) before filming them engaging Cave on subjects such as the reasons behind band break-ups, ringing Michael Hutchence to get through to Minogue and how to avoid fogging up the inside of your car.

"Structurally it was set up exactly as you would a feature film," adds Forsyth. "There was a script, scenes, locations, etc. The locations would be constructed, but the action within would be played more like a documentary." The result is like a documentary. But without being a documentary.

Pollard and Forsyth clearly put Cave through his paces, at one point subjecting him to 10 hours of psychoanalysis while they filmed. And this with a real psychoanalyst (an Alain De Botton lookalike) who digs deep and keeps Cave talking about his father's untimely death and his almost insatiable attraction to women with a certain look.

It is one of the touching parts of the film, though it maybe overestimates the audience's interest in what drives Nick Cave (daddy issues and Madonna/Whore complexes, as it turns out).

But the film doesn't really take itself as deadly seriously as all that. There are winks at fandom, at legacy-making and at the audience, too. There is a scene where we are treated to watching Cave open an early will he had written, fresh from the Nick Cave archive, complete with rumpled archivist. Pollard admits that the archive wasn't real, but the will and Cave's reading of it were. So there's the fantasy element, where the film-makers create the World Of Nick Cave and ask him to live in it.

"I'm sure we could have made a film like this about many other creative people," notes Pollard, "but because we have worked together with Nick, we had that trust."

She tells an amusing story about something that didn't make the film. Cave and Winstone, friends since Cave wrote the screenplay of The Proposition, which Winstone starred in, thinking their scene over, talked about getting something to eat. Winstone suggested fish and chips, but Cave demurred, saying, sincerely, that English fish and chips weren't very good. The resulting blast from Winstone was pure comedy, and the best demonstration of how the two men were with each other. Pollard says they had to cut this because it didn't fit the rest of the film, and scenes were curated carefully.

Another excision was Cave's wife, the model Susie Bick. Forsyth explains that they shot several scenes with her, but that they didn't work. Nevertheless, she fills the screen with her absence - much is made of "white face/black hair" - and is a powerful force for Cave, even if she is off-camera. Forsyth speaks amusingly about an American review which complained that no connection was made between Cave's ode to this colouring while standing in front of a large photo of Susie, who could have been photographed in monochrome, so white-faced and black-haired is she.

Pollard laughs and Forsyth says: "We wouldn't have Melvyn Bragg come out at that point and tell us what we were seeing!"

Pollard and Forsyth were keen to reveal what's beyond the showmanship that characterises many people's ideas about Nick Cave, and so touchingly, briefly, we are treated to a stick-thin, hair-slicked, skull-ringed and impeccably suited Cave, slouched low on a sofa, wedged between his twin sons, sharing pizza. Pollard says that is Cave to a T: "That pizza-eating guy is the exactly the same guy as onstage."

So it's all the more jarring when the audience is taken from the cozy environs of Casa del Cave and thrust wholesale into the electrifying presence of a bona-fide rock star, lit by a thousand lights and holding a thousand hands, midway through the ragged crescendo of Stagger Lee. It may not be a warts-and-all portrayal, but the film is an intriguing look at a part-manufactured idea of an interesting man. It's not all true, but it's no less genuine for that. The only voiceover is Cave's and his opinions are as opaque as ever.

Forsyth insists that to "show as well as tell" was particularly important to their approach in 20,00 Days On Earth. "A lot of documentaries feel they need to tell you everything," he says. "We wanted to show, be cinematic, in the telling of the story."

20,000 Days On Earth is in cinemas from September 19