Adaptation.

Being John Malkovich, the first collaboration between director Spike Jonze and writer Charlie Kaufman, was so ground-breakingly eccentric that Adaptation. doesn't quite come as a comparable shock; yet, for most of its running time, you can't entirely believe what you're seeing. The film is so audacious that the sight of two fat, sweaty Nicolas Cages arguing about the tenets of screenwriting strikes you as the very least of it.

Once again we're in the rarefied climes of high self-reflexivity. In Kaufman's script, Charlie Kaufman (played by Cage), the successful writer of Being John Malkovich, is commissioned to write a screen adaptation of The Orchid Thief, Susan Orlean's best-selling book about swashbuckling Florida horticulturalist John Laroche – a task that Kaufman, a quivering tub of self-doubt, finds next to impossible. Now, Susan Orlean is a real writer, The Orchid Thief a real book and John Laroche its real-life subject. Charlie Kaufman is a real person too, but his twin brother Donald, also played by Cage, isn't, even though the brothers actually share writing credits for Adaptation. itself. However, Orlean and Laroche also become fictional characters in Adaptation., vividly played by Meryl Streep and the saturnine Chris Cooper, both clearly having the time of their lives.

If its predecessor hinged on the paradox of John Malkovich being both himself and a ludicrous fictional version of himself, Adaptation. pursues such riddles even more obsessively. Apart from the fact that this both is and isn't an adaptation of Orleans' book (which Kaufman really was commissioned to adapt in the first place), there's the question of how much the film's real-life characters are or aren't themselves. You have to wonder what kind of deals the studio lawyers brokered with Orlean and Laroche that they agreed to be represented in a way that seems more intrusive than a supposedly realistic fictional account of their story might have been (for one thing, Kaufman shows himself having masturbatory fantasies about sleeping with Orlean).

You also wonder why Kaufman, supposedly the most publicity-shy man in Hollywood, chooses to lay himself open to public scrutiny, albeit as a self-obsessed onanist, in a way that screams for attention as stridently as a starlet's cleavage on Oscar night. Of course, just as we're wondering that, the on-screen Kaufman berates himself for his narcissism. As exhibitionistic comic abjection goes, Kaufman makes Woody Allen look an amateur: he ineffectually skulks around the set of Malkovich, sweats profusely whenever he talks to women, and sits hunched and helpless like a constipated bear over the blank page in his typewriter. (You can't help thinking a laptop would solve a lot of his problems.) Cage, the latest Hollywood hunk to ugly-up for a part, looks perfectly plausible as a blubbery, twitchy wreck with his plaid shirts and frizzy hair – like Gene Wilder on cortisone. What is strange, though, is just how easily and naturally Charlie and Donald co-exist on screen, thanks to computer effects and Cage's timing. Donald is Charlie's opposite: a party-hearty guy, a hit with women, and the writer of an inane-sounding serial-killer script, which needless to say proves a massive hit. Really, Donald is less a literal brother than a doppelgänger, as Brad Pitt was to Edward Norton in Fight Club; the joke, though, is that rather than looking like a smoothie hipster, Donald is Charlie's equally ungainly, badly-coiffed double.

Kaufman has another double of sorts – Orlean herself, as she struggles to write The Orchid Thief. That's right – he's having trouble writing a script about someone having trouble writing the book that he's adapting. Adaptation., though, is not just about turning books into films, but also about "adaptation" in the Darwinian sense, meaning the way species adjust to prevailing conditions to survive. Kaufman himself adapts by swallowing his pride and seeking wisdom from script guru Robert McKee – another real-life figure, played by a booming, belligerent Brian Cox – whose philosophy comes in 10 neatly prescriptive commandments. Not only does the fictional Kaufman embrace the McKee philosophy, but the last hour of Adaptation. – which, people had warned me, seriously goes off – goes off only because it comprehensively parodies the conventions that usually inform the third act of McKee-influenced movies. And yet Adaptation. itself is the very antithesis of McKee-ism, not least because it insistently breaks his golden rule: no voice-overs.

Adaptation. is a critic's dream, not because of the Hollywood in-jokes, but because it's genuinely and seriously about so many themes – not least, the experience of reading, of getting embroiled in a text and even falling in love with its author, as Kaufman does with Orlean. Most outrageously, perhaps, it's the first film (to my knowledge) that elevates the screenwriter to the world-creating auteur status that is usually a director's prerogative. Notice that I've barely managed to mention Spike Jonze so far? While his style is anything but invisible, somehow all the flamboyant touches – the Mapplethorpe-style montages of orchids, the potted four-billion-year world history – are folded so seamlessly into the film's overall concept that they don't look like flourishes at all.

Whether you find Adaptation. a complex exercise in auto-deconstruction, a rollicking showbiz farce, or excruciatingly self-indulgent (and why not all three, indeed?), here's a lesson it teaches that we might want to consider. Namely, that Hollywood's versions of real lives – whether Kaufman, Orlean and Laroche, or Erin Brockovich, Jake La Motta and Frida Kahlo – are at best questionable, at worst out-and-out horseshit. And now brace yourself for the inevitable deluge of manuals on how to write zany, self-referential Charlie Kaufman-style screenplays.

j.romney@independent.co.uk