The geeks have a new hero. Meet Watchmendirector Zack Snyder.

In a plush suite on the eighth floor of the Beverly Hilton Hotel, one of Hollywood’s most successful young directors is being gently teased by the international press.

Zack Snyder's last film, the mock-historical epic 300, grossed almost half a billion dollars. His new offering, the eagerly anticipated Watchmen, is an uncompromisingly dark superhero romp, exploring such grizzly themes as violence, sexual assault and erectile dysfunction, and features perhaps the most gratuitous exhibition of male full-frontal nudity (in a non-pornographic context) since Borat.

Yet none of that is what has the assembled press pack in a tizzy. Earlier in the week, billboards promoting Snyder's adaptation of Watchmenwent up all over Los Angeles. The original graphic novel, by Alan Moore and Dave Gibbons, was nominated as one of TimeMagazine's "all-Time 100 greatest novels", and the campaign to promote it is hardly excessive for a $100 million blockbuster. But eyebrows were raised at the film's tagline, prominently displayed above the title, which refers to Snyder as "the visionary director of 300".

The 42-year-old director might have attained godlike status among comic-book geeks and action nuts, but some critics remain stubbornly underwhelmed by his body of work. "A director of two films is suddenly a 'visionary' by his third" sniffed a headline in the New York Times. Wiredmagazine, meanwhile, speculated that the hyperbole over Snyder's film-making credentials might actually be intended to muffle word that the imminent Watchmenmovie is not quite up to scratch.

Happily, for a man whose films have been criticised in some quarters for being humourless and bombastic, Snyder in person is easy-going and self-deprecating. He takes the ribbing with good grace. “It’s embarrassing, of course,” he smirks. “But the marketing guys have a job to do, I guess.”

Surely he was consulted on the marketing strategy? After all, his own wife is executive producer. Snyder shifts uncomfortably. “Not . . . really,” he squirms. “They just came up with it and I thought, oh great, now I have this to deal with.”

Hubris could explain Snyder's elevated billing. But a more likely motive is simple economics. For all its bells and whistles, Watchmenlacks big-name stars. Tom Cruise, Brad Pitt, Jude Law and Keanu Reeves were all linked with the project at one time or another during its long gestation. But once Snyder came on board, he quickly opted, just as he done had on 300, to cast lesser-known actors.

Jeffrey Dean Morgan ( Grey's Anatomy), whose dark turn as the Comedian is one of the best things in the film, is here to meet the press. He exudes the cocksure attitude and easy charm of an actor who knows his star is on the rise. So, too, is Jackie Earle Haley, Oscar nominated in 2006 for his role in Little Children.Menacing onscreen as the right-wing vigilante Rorschach, he is shy and quietly spoken in person.

Only Swedish-Canadian starlet Malin Akerman ( 27 Dresses) strikes a bum note, drawing shocked gasps with a poorly chosen quip about the film's brutal rape scene. Her colleagues Patrick Wilson (Lakeview Terrace) and Carla Gugino (Night at the Museum) quickly jump in, insisting their co-star was only joking. But Ackerman seems unperturbed.

Overall, if the director set out to choose a cast whose star power would not eclipse his own arsenal of digital effects, then he has probably succeeded.

"Visionary" or otherwise, there's no denying that Watchmenpacks a hefty aesthetic punch. It is as rich and textured a spectacle as one would expect from the director of 300. Virtually every frame is crammed with minute details for fans of the graphic novel to pour over when the DVD comes out this autumn. The action scenes are also formidable. A prison-break sequence near the climax of the film – in which Danny Woodburn (Kramer's diminutive sidekick on Seinfeld) has a malevolent turn as crime lord Big Figure – is genuinely stunning.

Even some all-too-familiar rock standards ( All Along the Watchtower, The Sound of Silenceand the recently ubiquitous Hallelujah) sound reinvigorated when soundtracking Watchmen'salternate history of a 20th century in which the US wins the Vietnam war and Richard Nixon enjoys five terms as president.

The plot, meanwhile (and my there’s a lot of it), centres upon a group of costumed superheroes, the Minutemen, who unite to fight crime in New York in the 1940s. Only one (ahem!) member of the group, the supernatural naturist Dr Manhattan, possesses genuine superpowers, specifically an ability to “bend matter to his will”. All the rest of the troop can muster is an ability to ascend fire escapes with a tad more aplomb than you or me, so certain tensions were probably inevitable. The group disbands in the early 1950s, only to be reunited in an alternative 1985, following the brutal slaying of one of their number.

Attempting to summarise any further would be fruitless. For fans of the graphic novel, no synopsis could do justice to the breath of its vision. For the rest of us, well, the whole thing is frankly too preposterous for words. Costumed superhero dramas are inherently silly to begin with, but they only become sillier the more seriously they take themselves.

Snyder’s devotion to his source material, though, is slavish. According to some reports, he kept a copy of the graphic novel on his monitor at all times during filming, with the cast and crew referring to it as though it were a holy text. Although faithful to the spirit of the novel, the film’s ending is slightly different. That aside, the sheer volume of detail from the original novel that survives the transition to the big screen intact is extraordinary.

And yet, almost the first question Snyder is asked in every interview is whether he was nervous about the one minor change he did make.

“Of course I had some anxiety about changing the ending,” he confirms. “Honestly, I’m a huge fan of the Squid.” (The original ending involved a giant squid that . . . well, it’s a long story.) “But to keep the Squid in, I would have had to explain where it came from and that whole back story.” He remains, he assures us for the umpteenth time, a huge fan of the original novel.

What about the opposite question – why feel compelled to remain true to the original material at all? Historical dramas routinely feature composite characters and indulge in conjecture and outright invention; Frost/Nixon, for example, has the former president drunk-dialling David Frost in the middle of the night. Why are campy comic book heroes sacrosanct? It's just a film. Audiences will understand that.

Snyder shrugs. This isn't anything he hasn't considered a hundred times before. "My favourite example is No Country For Old Men. That's a frickin' Pulitzer Prize-winning novel. The movie is vastly different from the book and nobody gives a damn."

Could it be that the director is nervous about alienating Watchmen'snotoriously fanatical fanbase? "Absolutely," he agrees. "Because those guys are so vocal and, absolutely, they have the ability to do damage and shake opinion. They have to be acknowledged. You can't just write them off."

Perhaps the most fascinating thing about this film, then, from an outsider's perspective, is to witness the awesome power of online opinion. At every stage in Watchmen'sdevelopment, casting and production, fan-boy and -girl opinion was courted assiduously.

The first Watchmen trailer premiered at last summer's Comic-Con International in San Diego, the so-called "Super Bowl for geeks". When word leaked online that Warner Brothers were pressuring Snyder to stick with a two-hour running time, a fan petition called "Campaign for a three-hour Watchmenmovie" was launched online.

" Watchmenis a landmark work in the graphic medium," it stated. "It should be treated with the level of respect any great epic literary work is afforded, not like the average superhero movie." The finished picture clocks in at two hours and 43 minutes.

Back when Snyder was first announced as the director, fan communities were sceptical about his credentials. "These guys were saying, 'All he's made is 300, how in the hell is he going to do justice to this great Alan Moore book?'"

The greatest asset Snyder brought to the project, as he points out, was the artistic clout of someone whose last film made $456 million (€363 million). He is a genuine fan of the original text and had the power to get his own way.

"My attitude was, if I didn't make 300, there might be a Watchmenmovie. But it would be PG13, Adrian would die at the end, and there would be a sequel called Search for Manhattanwithin two years. I guarantee it."

The director has certainly struck a blow for the geeks. All that remains to be seen now is whether that translates into box-office success.

Watchmen opens today.

Watch and learn: how it got made

"We can't talk about the lawsuit, I'm sorry." Larry Gordon's opening salvo provides the first hint at Watchmen'stortured gestation. The veteran producer ( Die Hard, Field of Dreams) bought the film rights to Watchmenin 1986. On multiple occasions, the film was about to go into production, or so Gordon thought, only for a spanner to be thrown in the works at the last moment.

In the early 1990s, Terry Gilliam was attached to direct it for 20th Century Fox, with Arnold Schwarzenegger in consideration for the role of Dr Manhattan. After a series of script problems, Gilliam dropped out, agreeing eventually with the view of the graphic novel's creator, Alan Moore, that the Watchmenstory was "inherently unfilmable".

The project was revived in the new century, with co-producer Lloyd Levin ( Boogie Nights) signing up and screenwriter David Hayter contributing another script. Hayter was originally slated to direct, but in 2004 Paramount announced that it would produce his script, with David Aronofsky directing. When Aronofsky dropped out, Paul Greengrass replaced him.

The project only finally got off the ground when Zack Snyder expressed an interest in making the film. He brought in screenwriter Alex Tse to rewrite Hayter's script.

"It was Zack and Debbie [Snyder, Zack's wife and the film's executive producer] coming on board that got Warners interested," says Gordon.

The film eventually went into production, but not without further legal wrangling when Fox filed a lawsuit claiming distribution rights to the movie. The claim was upheld, to the surprise of many in Hollywood, and the matter is now believed to have been settled out of court.

Nonetheless, both producers express great pride with the end result.

"We are convinced that Zack has done the best possible Watchmenmovie that could have been made," says Levine. "It exceeds all our expectations."

"It's funny," he added, "but when you think about it, it's really only now that superhero movies are becoming mainstream. It has taken movie audiences this long to get up to speed with where comic books were 20 years ago."