Snyder was blunt in his response. "No way," he tells Metro. "I mean, am I masochistic?"

The 42-year-old director is a long-time fan of the novel written by Alan Moore and drawn by Dave Gibbons and knew how difficult it would be to turn it into a movie. But after the initial rejection, Snyder thought about it. As a fan, he knew it would be a tragedy to see anything less than a faithful big-screen adaptation. He began to visualise how he would make it. "I said: 'OK, I'll have a meeting,"' he recalls.

At the meeting with Warner Bros executives, Snyder began scrubbing line by line the studio's plan for Watchmen. Most directors would be shown the door but Snyder was holding a trump card.

While the negotiations were going on in 2005 he was in the editing room putting the final touches on 300, a stylised retelling of the ancient Greek Battle of Thermopylae. The studio knew from Snyder's unfinished cut of the swords-and-sandals epic that he was creating something special. Audiences around the world would soon agree. When 300, based on the graphic novel by Frank Miller, was released in theatres it was a critical and box-office sensation, earning a global take of $US456 million. Snyder made 300 for the relatively frugal budget of $US65 million, so it was one of 2007's most profitable films.

When Watchmen appears in Australian cinemas on March 5 it will be nothing like the studio's first idea. Snyder won the battle.

Watchmen is rated MA 15+ and is about 2½ hours long. It is set in 1985, as per the original graphic novel that comprises 12 issues, and it is sexy and politically charged. Watchmen tells the story of an alternative reality in which superheroes walk among us while Richard Nixon is serving his fifth term as president. The only character who possesses superhuman powers is Dr Manhattan, played by Billy Crudup. The story begins with the murder of a retired superhero, the Comedian, but another superhero named Rorschach believes someone is trying to assassinate his former colleagues. A convoluted story evolves.

"At 167 pages, I basically almost doubled the length of the script just trying to make it more like the book," Snyder says. "Although, to the studio's credit, when they saw my three-hour cut of the movie, they were pretty cool with me not knowing how to make it shorter.

"They weren't like: 'Bam, take that out.' They said: 'What the hell did you do? This isn't what we asked for.' But not in a bad way." Snyder is thrilled with the final product and, thanks to the studio, has been allowed to make the type of movie he always wanted to. "They have let me make a movie which is crazy in a lot of ways."

Actress Malin Akerman, who plays Silk Spectre II, describes the six-month Watchmen shoot, which took place in Canada, as "gruelling" for the cast but even more so for Snyder. "He has put everything he has into this," she says. "I don't know how he did it. Some days were 18 hours long and for Zack, he's the first one to get there, last one to leave and he never failed with his energy.

"He gave us all our energy when ours was fading. I think because it's such a passion for him, to see this novel come to life." Watchmen promises to be no ordinary comic-book fare. Take, for example, the soundtrack Snyder has created, using artists as diverse as Bob Dylan and Nat King Cole. Along with Cole's classic Unforgettable, Snyder also uses Dylan's anthem The Times They Are A-Changin'. "I love it because it's strange to see a song like that in context with a superhero movie," he says. "You really feel how far away you are from Fantastic Four. It's a pretty long way down from there."

Actor Matthew Goode, who plays Ozymandias, says he always knew the film was safe in Snyder's more-than-capable hands. "Zack is verging on being another Ridley Scott. It's a bit early to be saying that but 300 certainly opened people's eyes to what he can do. Watchmen is an R-rated movie [in the US] and it's dark material.

"Not many people know about Watchmen but those who do know it absolutely love it and we've all tried to do our bloody best and have given this our all." WATCHMEN Director Zack Snyder

Stars Patrick Wilson, Billy Crudup, Malin Akerman, Jackie Earle Haley

Rated MA 15+.

Out March 5.