Easing himself into a chair at LA's landmark Bel Air Hotel, Leonardo DeCaprio is looking very little like Howard Hughes. With his backwards LA baseball cap, washed-out blue Polo sweatshirt, baggy Levi's and scruffy boy-beard, he bears scant resemblance to the stylish Hollywood playboy/producer/aviator he's captured so perfectly in The Aviator, Scorsese's homage to the American legend.

As Scorsese himself recently told Vanity Fair, maybe it's to do with DiCaprio's uncanny ability to "shape change": today he appears to have shape-changed into the Young Hollywood, posse-loving, supermodel-dating Leo we're more used to seeing staring out from the pages of the weekly pap-rags.

Today is also just the one day before his 30th birthday - "I've got another eight hours not to think about it," he says - and he seems far from excited about getting another step closer to full-blown adulthood.
"I'd probably rather still be 20 to tell you the truth," he admits. "I don't want to be 30. I'm still a juvenile in some ways and kind of mature in other ways. I feel I've just accumulated more information and have learned more things."

One thing it's clear the reluctant grown-up has learned is how to spot a good movie role when he sees one. Having initially stalled on his post-Titanic career - after a two-year hiatus, turns in 2000's The Beach and 2001's Don's Plum didn't exactly find him capitalising on his elevated status - his more recent teaming with, first, Spielberg on 2002's Catch Me If You Can and then Scorsese on Gangs of New York has garnered him all-important credibility points.

Scorsese likens DiCaprio's process to that of another of his personal favourites, Robert De Niro, and DiCaprio - already slated to film a third movie with Scorsese in 2006, The Departed - feels he's struck Directors Gold.

"The appeal is that I'm working with a master of his craft," he explains, "and I trust nearly everything he says in terms of what to put up on the screen because he's like a professor of cinema. You're working with someone who is at the top, utmost tier of their craft."

Scorsese's craft has rarely seemed more fluid and energetic than in The Aviator, the story of a man whose life was initially guided by three naked ambitions: to become the richest man in the world, to become the biggest producer of movies in the world, and to become a pioneer of aviation. Howard Hughes pretty much became all three, picking up the tag of 'world's biggest playboy' and, ultimately, 'world's strangest recluse' along the way, something DiCaprio could certainly relate to.

"Sure - I feel like hiding all the time," he grins. "But I never do because I don't let myself be a victim of this ridiculous thing that is 'being recognised'. There are a lot worse things in the world that people have to deal with. It's not like being famous is a civil human rights issue that people should be concerned with. Who cares about us? We're lucky bastards."

After a charmed beginning to his own life, Hughes wasn't quite as lucky a bastard. Born in Texas and into money in 1905 - his father had invented a diamond drill bit that revolutionised the way oil prospectors operated - young Howard's triumvirate of ambitions began in earnest at 18 when he inherited the business after his father's death in '22.

Oil was never Hughes' thing, so he headed for Hollywood and while his company continued to pump oil, he began pumping the profits into movies, and not with a little success. He made a star of Jean Harlow, smashed box office records with Hell's Angels - at $4million, the very first big-budget movie - had two films nominated for Oscars (The Racket in 1928 and The Front Page in '31), and managed to get the censors all hot and bothered when he invented a cantilevered push-up bra for the already generously endowed Jane Russell in The Outlaw.

Ultimately, despite the profile and access to the period's hottest Hollywood tottie - he famously dated Katherine Hepburn, Ava Gardner and Jane Russell (take that, Dicaprio) - the land of make-believe was not enough for Hughes, and he left movies behind to indulge his other passion: aviation. As driven to fly as he was to make films, his aviation successes were enormous. He set many world records, including, at 7 hours, the fastest time travelled between Los Angeles and New York; he built out of wood what is still the world's largest aircraft, The Spruce Goose, which is more than 50 percent longer than a Boeing 747; and he flew his own H-1 airplane a record-breaking 352mph. Along the way he nearly killed himself piloting a test flight - he spent nine months in hospital recovering from death-defying injuries - and bought and ran passenger airline TWA.

"I think he's someone that took things to extremes," understates DiCaprio. "I think he literally had no moral high ground, no parental figures as a young man, had all the money in the world, and made a laundry list of things he wanted to do. He had nobody to tell him what he was doing wrong, and I think he sacrificed his own happiness ultimately in order to achieve these dreams."

Despite the realisation of many of his dreams, sadly Hughes is perhaps best known for the last 20 years of his life, when he lost his battle with hypochondria, germophobia and obsessive compulsive disorder, and became a urine-bottling, toenail-growing, fully-fledged weirdo recluse living in a hotel room in Las Vegas. By the time he died in 1976, age 71, his appearance was so drastically changed and he'd been seen by so few people for so long that the Treasury Department had to use fingerprints to identify his drug-addled body.

Ultimately, his is a lifetime of stories that Scorsese could have (and Peter Jackson probably would have) chosen to dedicate a trilogy of films to: the Hollywood Years, the Aviation Years and The Recluse Years. Instead, The Aviator covers the period between 1928 and 1948, introducing the viewer to the young, handsome and energetic producer as he struggles with spiralling budgets and "not enough clouds" on the set of his World War 1 epic Hell's Angels. It finishes with his very public battle with congress - Hughes spent a lifetime battling authority figures over everything from censorship to taxes - as he attempts to dismantle the government-mandated monopoly held by Pan Am over international travel routes.

"I think the fact it focussed on Howard as a younger man is the reason the movie got made," DiCaprio suggests. "It follows all the successes that he had in his life. It's not very cinematic to see a guy locked in a hotel room sneezing and making business deals."

"For me aviation was the spine," adds screenwriter John Logan. "I believe it was the entity in Howard's very crowded life that he cared about the most."

Though it wasn't DiCaprio's first understanding of who Howard Hughes was; like many, he too thought of him as "this old man with Kleenex boxes on his feet and long hair". Until, that is, he read Peter Harry Brown's 1996 Biography, Howard Hughes: The Untold Story.

"It was then I realised he was a real pioneer of American history as far as aviation is concerned, and how he was the first independent producer to make a four million dollar picture and make one the most sexually explicit movies in The Outlaw and one of the most violent films with Scarface. I learned how he challenged the censory boards and how he had really hardcore obsessive compulsive disorder and how he was one of the biggest Casanova womanisers of the last century.

"That's when I knew he was a multi-dimensional, fascinating character that would be exciting to play."

Convinced of the validity and depth of the role, DiCaprio set about meticulously researching it, relying enormously on TV footage covering the senate hearings of his battle with Pan Am.

"I literally got to watch his movements and see how he really was in a public form. But the biggest worry was how the germaphobe stuff would work. That took a lot of research and a lot of effort to really understand what it is that makes you afraid of, say, shaking hands. I didn't understand what the heck that was so I worked with a doctor and a couple of people who had OCD to try and really give an accurate depiction."

"Leo did an incredible amount of committed research, the like of which I've never seen a film actor do before quite frankly," says Logan.

DiCaprio, however, wasn't the only one that had to bury themselves deep into the nuances of a non-fictional character. Cate Blanchett took on the scene-stealing role of Hughes' ex, Katherine Hepburn, and trawled her way through many of the late-actresses films and appearances, striking gold when she stumbled upon a television interview she'd done in 1974 with US TV host Dick Cavert.

"She was a much older woman," she explains of the clip, "and hearing her speak and not speak someone else's lines - I know very well the difference between one's performance persona and who one actually is. It's often miles apart, so I was trying to walk that line and work very closely on her vocal mannerisms."

Meanwhile, Kate Beckinsale, in the role of Ava Gardner - another of Hughes' paramours - had a tastier way of digging into her character.

"I did it with chocolate," she explains. "I'm not really the right shape for Ava Gardner in real life so I gained a stone and a half, and I was actually quite cheered about how much chocolate it took to gain it. Now, though, I couldn't look at a peanut M&M again if you paid me, which is a miracle. But I managed to gain weight on my face and my boobs and my bum and everywhere. On the set it was fine, but I'd go home and feel like a big fat bastard."

Rotund illegitimate child aside, for all of the film's stars the making of The Aviator was almost as exciting a journey as one of Hughes' record-breaking flights.

"I found the journey, the drive of the film, really fascinating," says Blanchett, "and Marty's really captured that fast talking, witty energy that the films of the 30s and 40s had."

"Before making the film, I didn't realise how much Howard Hughes had achieved," adds Beckinsale. "I knew him much more for being a nutter. Which is really sad because if someone's got that kind of brilliance there's often a flip side that goes with it. And it's a shame when people know a bit more about the flip side than about what you actually did."

It's clear that The Aviator could go some way to diminish the worldview of Hughes as a loony recluse being stabbed to death by his own toenails and help replace it with a more fitting epitaph. And both Beckinsale and Blanchett - along with No Doubt's Gwen Stefani, who's featured in her first ever movie role as Jean Harlow - agree that it's in no small way due to the generosity and skills of their director.

"Scorsese couldn't have made me feel more comfortable and like I was meant to be there," admits Stefani, who was initially overwhelmed with nerves at the prospect of working on the film. "He was so passionate and I love seeing people like that; he knew everything about Howard Hughes and the time period."

"He's incredibly detailed and a perfectionist and very sensitive aurally and visually," adds Blanchett. "If something's offending him on set visually it means it distracts him and he can't concentrate, like if there's a noise at the other end of the studio - it's like he's got the hearing of a dog. Therefore he's really sensitised to performance and can really pick things up that maybe a lot of others would miss. At the same time he's a very energetic man and so he's interested in the performance energy. It was fascinating to work with him."

The man that perhaps knows him best puts it more succinctly.

"He's an obsessive movie nerd," DiCaprio says, laughing. "And he relates everything in his life to movies - it's really amazing. And it's great to be around somebody who loves what they do that much. He obsesses about films: he doesn't go for walks in the park when he has a couple of hours off, he goes into his screening room and watches another movie that he's seen 30 times already. The man's amazing."

'Amazing' is a recurring word bandied about by those closest to the director: unfortunately, 'Oscar' and 'Scorsese' are two words still not seen in the same sentence, despite overwhelming classics like Goodfellas, Raging Bull, Taxi Driver and Mean Street making an appearance on his CV. Could The Aviator change things?

"I really would love for Mr Scorsese to get an Oscar, to tell you the truth," says DiCaprio. "Because I think his contributions to the cinema are unprecedented, and the fact that he hasn't won yet is like a cruel practical joke."

Any hankering for a little gold mantelpiece decoration of your own?

"Of course it would be nice," he admits. "Any actor that would say they wouldn't want that would be... well, why would they be in the business if they wouldn't like to have somebody say you did a worthy enough performance to get nominated? Of course they would like that."

Certainly, two of his Aviator co-stars are sure DiCaprio would be a deserving recipient.

"There's a relentlessness to him that he's prepared to keep going and going and going and going until he feels he's got it right," says Blanchett of her co-star. "He's really committed to every aspect of it. And not just his own performance - the whole thing was important to him."

"I think he inhabits what he's doing," adds Beckinsale. "I thought he'd be all Young Hollywood, but he was nothing like that - just incredible. I was very surprised - he seemed very grown up."

For birthday-boy Leo's sake, though, hopefully not too grown up.