Puzzle power

HE'S been seen riding the range in the gay western Brokeback Mountain and joining the American marines to fight in the Middle East in Jarhead. Now Hollywood actor Jake Gyllenhaal is following the trail of clues left by a serial killer in his latest screen role.

He stars as real life political cartoonist turned detective in Zodiac, the new film from Fight Club and Seven director David Fincher about the murderer who terrorised the San Francisco Bay area for several decades from the late 1960s.

Gyllenhaal's character, Robert Graysmith, was drawing cartoons on the San Francisco Globe when Zodiac began to send letters containing clues to his identity and the killings to the newspaper.

A puzzles addict, the cartoonist was intrigued by the coded messages, gradually becoming more and more involved in sifting through the evidence to try and identify the killer whom the police had failed to find.

The film is based on Graysmith's book about the Zodiac case.

Gyllenhaal knew nothing of the story when first approached to be in the movie. It was Fincher's participation that interested him, so he read early drafts of the script.

"The first draft was very much a sort of cop thriller with your typical characters and I thought it was fascinating," he says. "The murders terrified me and, just thinking of David doing the movie, I was totally in.

"A couple of months went by and he started working on the script and I read a draft that he'd done and it was 200 pages long. It was this sort of opus and the murders were still terrifying, and yet there was still all this character stuff in between. All of it was real stuff that actually happened. I just thought, this is amazing'."

As an actor, he found it was less about him obsessing with the facts as understanding why Graysmith found it so fun. "That's what's hard for people to understand - that this man enjoyed searching for this person, whereas a cop would have done it because it was his job, or a journalist would have done it as it was his job," says Gyllenhaal.

"He really had fun. It was like a kid at a candy store, which I thought of as an interesting duality or an interesting juxtaposition to your normal detective story."

Fincher's previous films are noted for their visual style but Gyllenhaal feels he wanted to do more for the actors in Zodiac. "He gave us a space in which we could work," he says.

"But he knows what he wants.

He's very clear about it and in a lot of ways that discipline's like working on Shakespeare. You have to stay within the iambic pentameter, you have to stay within the rules, but within those rules I think are amazing discoveries."

Mark Ruffalo co-stars as one of the leading detectives on the case with Robert Downey Jr playing a Globe crime reporter who also gets too involved in tracking down the killer. Gyllenhaal recognises that Fincher has put together an unlikely trio of actors.

"They're highly experienced, much more than I am, and I was learning from them every day," he says. "Particularly Robert. He's sort of like Tinkerbell, the court jester dancing around you all the time but constantly inspiring.

"Just imagine Robert with buckets and buckets full of inspiration, handing them over to everybody he works with. And as an actor, it's extraordinary."

Making the film was taxing with a 110-day schedule where "you're talking most of the time and it's a story about finding a serial killer", as he puts it. "It was gruelling in terms of the mindset you had to be in."

Some people revel in the process of working under Fincher's methods, other are disturbed by it.

"But the audience gets to experience it for two- and-a-half hours, which is a big jolt to you when you're in that world," he says.

"I don't know if I would ever want to go back into the Zodiac world again, but it's quite a world to be in."

The real Graysmith says it's thrilling being playing by Gyllenhaal on screen. He also pays tribute to Fincher's vision and his insistence on realism in the San Francisco Chronicle set.

"What Fincher does is he goes and gets the blueprints, 3D graphics. I'm looking on the screen, it's alive again. For the movie, it's a block-long set. And you open the drawers - Chronicle notebooks, Eagle pencils, the vacuum tubes work, the phones work. The movie shows the lighting system, which is extraordinarily unique. He has recreated it. Nobody in the world would remember. I just think God, that's incredible that he went to that kind of trouble."

In retrospect, Graysmith can see how obsessed he became with the case. "But when you're in the grip of it, you don't really know," he says. "Now that I get thinking back, I get a little bit more lucid about it.

Everybody was pretty much obsessed by this because we really hadn't run into anything like this before. We had Son of Sam, Boston Strangler and then Jack the Ripper.

But this is something entirely new back in 1968 that we really hadn't dealt with before."

* Zodiac (15) opens in cinemas tomorrow