Paul Thomas Anderson brings Pynchon to the big screen for the very first time

So it seems apt, somehow, that his latest film - the seventh of his career - is Inherent Vice, the first ever movie adaptation of a novel by the great American writer (and publicity-averse) Thomas Pynchon. The man behind such titanic novels as Gravity's Rainbow and Mason & Dixon, Pynchon has deliberately shunned publicity ever since he published his first book, V, in 1963. That he has managed to stay below the radar in the age of the internet and mobile-phone camera is nothing short of remarkable.

I don't know how he does it, I say to Anderson. "I don't either, but more power to him. It makes everybody so curious - and me as well." The obvious comparison is with filmmaker Terrence Malick, the director of Badlands and The Thin Red Line, who similarly has chosen not to be a public figure, refusing to engage in any sort of publicity, even the obligatory on-set photographs. Anderson calls them "people who have taken what I would think is the high-road". Would he ever consider doing it? "It's too late!" he cries. "I don't have the guts to do it. I would love to do it."

Perhaps it is too late for Anderson, who has been reluctantly in the public eye ever since his first film, gambling drama Hard Eight, came out in 1996. Since then he's been nominated six times for Oscars, including a well-deserved Best Adapted Screenplay nod for Inherent Vice. He knows he needs to show his face. "We're making films that need help. We need to spread our word. We need to get people into the theatres beyond just the people we know are our hardcore aficionados who will show up."

In truth, the 45-year-old Anderson, dressed today in a creased white T-shirt and jeans, doesn't come across as a man uncomfortable in the spotlight. The night before, he introduced a screening of Inherent Vice at London's Prince Charles, a suitably crumbling rep cinema. But, typically, it's a 'secret' screening. There's something exciting about finding out about this by chance, rather than having it splurged all over the unforgiving web; it's reminiscent of the covert societies that fill Pynchon's conspiracy-laden books.

Adapted during the period when the director was also working on his last film, the Scientology-inspired drama The Master, Inherent Vice may not be the profound Pynchon novel Anderson could've chosen, but this 2009-published hippie-detective yarn was certainly one of the more accessible. "Maybe there were books [of his] I loved even more, but this somehow encapsulated a lot of his worlds, like he touched on in other books. It seemed like a great opportunity to, yes, use the book, but really use the book to speak to his whole in some way." He pauses. "Maybe that's too big a thought."

Certainly Pynchon's interest in counterculture is prominent, as his antihero, the pot-hazed private eye Larry 'Doc' Sportello (played by a hairy, unwashed, Neil Young-inspired Joaquin Phoenix) goes on an increasingly baffling mystery after his ex-girlfriend (Katherine Waterston) comes to him for help. Set in 1970, the moment when the Sixties burnt out and free love evaporated in the wake of the Charles Manson murders, both Pynchon and by extension Anderson show a world where ideals are crumbling in the face of fear and paranoia.

Doc is frequently referred to as a 'hippie' in the most derogatory of ways. "I was shocked when I first read the book - 'f***ing hippie!' It was constant!" says Anderson, but he estimates Pynchon got it just right. "Initially, it was like looking animals in the zoo... 'Oh, how interesting, look at those hippies!' But then Charles Manson came along and it became, 'These f***ing hippies are going to murder us.' They became synonymous with something that was beyond their control."

In a strange quirk of fate, Anderson was actually born on the day the Sixties were officially over - January 1, 1970. "My parents weren't hippies," he says, "[even though] my family had just moved to California in '68 or '69." Anderson's father Ernie, in fact, was an actor and the voice of US television network ABC, not to mention the host of a late-night Cleveland television horror show, under the moniker 'Ghoulardi' (a name Anderson later snagged for his production company).

If Anderson had been born 20 years earlier, would he have been a hippie like Doc? "I presume so, yeah. I guess. I probably would've had long hair, and liked that music, and that dope and all that." While drugs have featured in Anderson's films before, it's usually been the more destructive forces of cocaine or crack than the gentle waft of weed, something that gives Inherent Vice its woozy rhythm, as characters drift in and out of Doc's orbit like foggy hallucinations.

Inevitably, talk turns to Robert Altman - Anderson's all-time hero and a huge influence on his work. It's almost impossible to watch Inherent Vice and not think of Altman's The Long Goodbye, his seminal take on Raymond Chandler's gumshoe Philip Marlowe, relocated to 1970s LA. "If anything, I felt like I had to forget The Long Goodbye. I obviously love that film. But for once in my career, I obviously didn't need to rip off Altman!" His mission was clear: set it aside and "focus on what was clearly Pynchon's obsessions, and make it about him".

Naturally, this being a Pynchon-related project, rumours have circulated. Joaquin Phoenix has said that Anderson regularly talked to Pynchon, while there are unsubstantiated claims that the author has an uncredited cameo in Inherent Vice. Anderson, of course, denies all knowledge of everything. "I don't even know if he even exists," he says, playfully, happy to fuel the mystery surrounding a man who was once believed to be one and the same as the equally reclusive JD Salinger.

While Anderson writes all his movies solo (and five of his seven films fall into the category of 'original', not adapted from any source), he doesn't hold what he does in high esteem. "This is not any kind of modesty. Screenwriting is not real writing. It's not. It's hard work, and you can be good at it or not good at it, and it's challenging. But it's a thing to get you to the set to make the movie. When I look at Pynchon, when I read his books, I'm in awe."

Like any artist, Anderson suffers from doubts. I ask if he's ever considered writing a broad comedy, like Bridesmaids (which his wife, actress Maya Rudolph, featured in). "I thought this was that!" he replies, looking wounded. "You go into something with the intention [of] 'Let's go and make a broad comedy', and it really doesn't turn out like that at all. It turns out like this." He needn't worry: Inherent Vice is still funny. It's a comedy in the way his 2002 film Punch-Drunk Love was a romance, in the most warped of ways.

Whether others agree remains to be seen. Before Christmas, in the US, the film grossed an impressive $330,000 from just five theatres across its limited opening weekend - but after opening wide in January, it's so far only gone on to make $6.4 million, suggesting it hasn't gone beyond the aforementioned "hardcore aficionados". Despite a to-die for support cast, including Owen Wilson, Reese Witherspoon, Josh Brolin and Benicio Del Toro, it was always going to be a tough sell.

Anderson jokes about changing the ending, to deliver those more belly-busting comic beats. "That's what I'm talking about - get some of that box-office!" he laughs. Of course, just as Pynchon is never going to write an airport novel, so Anderson doesn't have it in his DNA to turn in a mainstream studio comedy. Even if he does admire the fart gags that occasionally pop up in Pynchon.

Upon leaving, I shake his hand and he leaves me with a brief plea. "Spread the word, please," he says. I'll do my best, I reply.

Inherent Vice opens in cinemas on January 30