Watch the opening five minutes of the film version of Alan Moore's dystopian comic book. Plus, director Zack Snyder fights back at his critics and reveals how he almost cast Brad Pitt. Watchmen is out on DVD and Blu-ray on 27 July

It could have been so very different: Christian Bale as Dr Manhattan, the cyan superman of the Watchmen universe, Brad Pitt, perhaps, as Nite Owl, the liberal face of masked vigilantism. Who knows? Perhaps Angelina Jolie could have portrayed the slinky yet vulnerable Silk Spectre. Tom Cruise, in Collateral-style sociopath mode, might have made a passable Rorschach.

Zack Snyder is talking about an early conception of Watchmen, his adaptation of the seminal Alan Moore graphic novel, in which the various characters were to have been played by A-list Hollywoodlanders. The idea was to use the celebrity status of the actors to mirror the obsessive public scrutiny experienced by Watchmen's "masks", who exist in an alternate 1985 in which superheroes - of a sort - have been walking the streets for the past half century.

"It's funny because early on we talked about doing a bigger, more sort of Ocean's Eleven style cast," says Snyder, on the phone from LA. "But the problem was that, as I was working on that concept, it was all about the irony of casting a movie like that, with big stars, so that the casting kind of commented on their roles.

"The truth is that it's a difficult thing for actors to be that self aware. I think in the end it's a perfect cast because they are those characters. I'm not sure it would have worked with, you know, Brad Pitt in the Nite Owl suit, or whatever. When you have people on screen that the audience doesn't know so well, the characters have their own identity: it becomes its own thing."

Snyder at the Berlin premiere of Watchmen. Photograph: Johannes Simon/Getty Images

And that's also what's noticeable about Snyder's version of Watchmen, out on DVD in the UK next week. It too has its own identity, one which transcends its roots in Moore's original comic book. From the glorious, hyperreal montage that comprises the opening scene – as Bob Dylan's Times They Are A-Changin' serenades 50 years of alternative US history where masked vigilantes have changed the course of the 20th century – to the climactic denouement, rather different to Moore's (pretty bonkers) ending, the film is resolutely Snyder's own. Just as the original graphic novel represented a sea-change in comic book sensibilities, Snyder's film bears little resemblance to any other comic book adaptation of recent times.

That may have been its downfall with the critics, who were not always kind, and it certainly didn't help the movie's box office, which failed to meet expectations of a giant, Dark Knight-style haul. Yet few could criticise Watchmen as the sort of hack job expected from a former commercials director with only two previous features under his belt (a remake of zombie classic Dawn of the Dead, and another comic book adaption, the notoriously gory 300). A significant minority labelled the movie a flawed work of genius.

"The thing I find fascinating about the whole way Watchmen was received is that 10% or less of the critics seemed to have actually read the graphic novel," laughs Snyder. "I feel like a lot of them just went to Wikipedia. Because it really is not a movie, in a traditional sense. And if you try to analyse it in those terms – and not in terms of its relationship to pop culture – then you kind of miss the point.

"It's a two-and-a-half hour R-rated movie, and there's no precedent for that type of film becoming a huge blockbuster. What's popular about The Dark Knight is that it's a superhero movie at its core. When Batman puts on his costume, that's badass: 'Yeah Batman, go kick some ass'. Watchmen is an entirely different experience: it punishes the audience. It says: "Oh you like the Comedian? Oh, he's a rapist, by the way." From an intellectual standpoint that's fun to do, but its offputting if you're there to enjoy a movie that's supposed to be a superhero movie.

"At the same time, I really wanted it to be marketed that way. I wanted people to think it's going to be a standard superhero movie, and then they're confronted by all these ideas. Because that's what the graphic novel did to me when I read it. Someone said to me: 'Hey you have to check out Watchmen, it's really cool.' And I read it, and I remember thinking: 'OK, this is going to be a cool graphic novel, with superheroes.' And then half way through - well less than half way - I found myself thinking: 'What's this? What's happening here?' And that was a cool experience for me, especially where I was in my graphic novel education. So I tried to bring that into the movie as much as I could."

Watchmen Photograph: PR

One area in which the film version surpasses the occasionally twee source material is in its all out action sequences, which are unrelentingly mucky and mesmeric, but surprisingly classy in their realisation. Snyder's trademark slo-mo blends in nicely and there are no obvious, cringeworthy moments reminiscent of the classic "This is Sparta" sequence in 300. Along with the film-maker's bloodthirstiness, it's an aspect of his work that has seen Snyder criticised in some quarters. Is that something that bothers him?

"I wasn't just going: 'Oh we need more slo-mo here,'" he laughs. "I don't have a sign or anything: 'More slo-mo!' I actually really restrained myself this time.

"It's a little bit of grease - it kind of smooths everything out and makes everything look a little more graceful," he adds. "The fun thing about Watchmen was to try and make those things that I love part of the movie, to make those techniques comment rather than just exist on their own as a cool device. I hope that's what I did, because I felt like I was objective."

One thing Snyder can be justly proud of is the performances he drew from the cast of Watchmen. Yet the director is happy to admit that the likes of Jackie Earle Haley, whose take on the morally absolute Rorschach brought him huge acclaim, and Billy Crudrup (Dr Manhattan), were so well-prepared, they did not require significant direction.

Watchmen Photograph: Photo courtesy of Warner Bros. Pictures./PR

"I think Jackie did an amazing job," says Snyder. "I can't imagine anyone else being Rorschach. He cared so deeply about the part and about the character, that once he and I had had conversations about what he wanted to do, I was confident. It was kind of a case of that was taken care of. He's a very challenging actor in the sense that he wants everything to be perfect. In a movie you have a number of takes and a schedule, but you often want one extra take. And then he would nail it."

I suggest that Crudrup's task, to inject life into the omnipotent Dr Manhattan despite the character being realised entirely via motion capture techniques, must have been particularly tough.

"With Billy I knew he was an amazing actor, but he really gave the animators everything they needed," says Snyder. "They looked at his performance and just duplicated it. And it was awesome. Dr Manhattan is probably my favourite character, so it was difficult that it was a labour of love. You make your whole movie and then that performance is only revealed at the end of the process. I knew Billy had done it, but it was a case of: if they can get Billy in the movie then it's going to be awesome."

While his cast's professionalism may have been a boon, Snyder's task on Watchmen was not helped much by the looming ghost of Moore, who maintains something of a reputation as a surly Northampton hermit. The writer who transformed the 1980s comic book scene with graphic novels such as V For Vendetta and From Hell condemned the movie out of hand before it had even reached cinemas, claiming his original work was unfilmable. Did Snyder try to reach out to the former 2000AD man?

Watchmen Photograph: PR

"When I came on board this movie he had already sworn us off," says the film-maker. "I didn't even get a chance to plead my case, to be honest. I have great respect for Alan and he had asked: 'Please don't try to approach me or talk to me or change anything about what I think.' So really I just tried to respect that as much as I could. And the problem with that, was that it basically just meant: don't ask. He's clearly a genius, and I hope – I'm sure he doesn't, but I hope – he understands; I was just trying to respect his wishes. He's actually been amazingly cool about it recently."

Yet this does not sound like the Alan Moore who, prior to its release, told a journalist from the LA Times that he had put a curse on Watchmen, adding: "I can tell you that I will also be spitting venom all over it for months to come."

"Well not cool, but not like lashing out at us," backtracks Snyder, chuckling. "I'm sure he's still like: 'I'll kill that Snyder', but maybe it's a boring question now or no one's asking him it."

I tell him I have a sneaking suspicion that Moore might actually quite like the film, if he saw it. "I don't know if he's seen it, so I can only speculate," he says, tactfully.

One suspects that part of Moore's problem with the film was that his original book is not a linear work that lends itself to an orthodox movie plotline. It is a colourful scrapbook of different stories told through a variety of media: excerpts from the memoirs of former superheroes, cuttings from news articles, even an entirely separate but intertwined story in the shape of bloodthirsty pirate comic Tales From the Black Freighter. These all came together to form a vivid, post-modern take on comic book tropes that both celebrated and satirised the genre and its medium. The theatrical version, despite its epic running time, could never hope to equal that sort of depth and richness.

Fans are still hoping that the eventual "Ultimate" cut, which will follow a three-hour plus director's cut onto DVD (the version about to be released is the theatrical version), will finally present Watchmen as it was meant to be seen, complete with regular segueing from the main story into the Black Freighter subplot, and the double-act between a comic-book obsessed young boy and a newsstand owner (both named Bernie), which are as important to Moore's version as the main storyline.

Watchmen Photograph: PR

"I made a deal with the studio that I would do The Black Freighter section [for a separately available DVD] as long as they gave me some money to shoot the ins and outs with the two Bernies at the news stand," says Snyder. "With those two actors, we almost did a separate movie. They didn't even know that we were making the whole Watchmen movie. As far as they know the whole thing takes place on a street corner. I think that [for] fans of the graphic novel, when they see the ultimate version, it will complete a bunch of the storylines."

Of course, any critics who were confused by the original movie are going to really hate this version, but Snyder, again, doesn't seem to be too bothered. This is a film-maker almost uniquely in touch with his audience: he doesn't come from an arthouse background, but then neither do most of his viewers. He doesn't particularly care whether he is lauded as a great director by the kind of critics who love to watch arthouse movies.

"I guess I like gore and action. I like genre," he says. "I make the kind of movies that I would like to watch."

Snyder doesn't get nearly as much stick as another former commercials director who made the leap into film-making, the much-maligned McG. Does he feel there is an unfair stigmatism attached to those who launched their careers in commercial territory?

"I'm really proud of the work that I did in the ad world," he says. "I really feel like it was an incredible visual school for me. I did 15 years of commercials, three a month, a lot of them in Europe. I'm a huge fan of arthouse and independent film-makers, but it's hard to compare that with 15 years of me running film through a camera every day, so that the tools are second nature. You can say what you want about me as far as storytelling, but shot-making is a thing that I feel pretty comfortable doing.

McG. Photograph: Graham Whitby-Boot/ Allstar

"McG is a really nice guy but I think he's made such an eclectic span of films that I can't say that anyone really has a handle on what he's about. I just make movies that I like, and that I want to see. I do think that commercial directors do get a bad rap. Everyone assumes they are just going to be very Hollywood and just want to crack out the blockbusters. Maybe it's because I've made slightly odd films that I've gotten around that a little bit."

Watchmen certainly makes for a pretty odd sort of superhero movie. But then the graphic novel was a pretty odd sort of comic book. Hollywood would no doubt have been pleased if the film had ended up being the Ocean's Eleven of superhero movies that Snyder once considered. Instead, Watchmen turned out to be something far less generic, a lot less facile and, I suspect, rather more durable. Even Alan Moore might approve of that.

• This article was amended on 29 July 2009. The original referred to the "I am Sparta" sequence in 300, and asked if there was unfair "stigmatism" attached to directors who began by working in the commercial realm This has been corrected.