Mark Ruffalo wasn't interested at first. The way he saw it, Zodiac was just another true-crime mystery flick – a genre he wasn't particularly keen on pursuing.

But then, the 39-year-old actor had a change of heart after seeing a revamped script. Zodiac also stars Jake Gyllenhaal and Robert Downey Jr.

On second read, Ruffalo says, he suddenly saw the potential of Dave Toschi, the former San Francisco Police Department detective who became embroiled in the now-infamous Zodiac serial-murder case, which claimed four lives in the late '60s and put Northern California on high alert.

Although several suspects emerged over the years, most notably Arthur Leigh Allen, who died in 1992, the case was never solved.

“[Director David Fincher] and [screenwriter] Jamie Vanderbilt breathed such depth and nuance and life and detail into Toschi,” says Ruffalo.

Of course, Toschi's life was rich for mining and already had been loosely immortalized on film. Steve McQueen based his character on Toschi in the 1968 thriller Bullit ; Clint Eastwood also modeled his Dirty Harry character after the veteran detective.

Ruffalo, who met the real-life Toschi, says he was intrigued by his innate “grace,” even later in the face of scandal.

Accused of forging a letter that the Zodiac killer supposedly sent to the San Francisco Chronicle , Toschi was forced to defend his reputation and job (he was eventually cleared of the charge).

Ruffalo says Toschi, who now owns a Bay Area private security firm, is “a total gentleman who's been wounded” and still remains haunted by the case's unsolved status.

“He handled it all with dignity even though he never fully recovered – his career was done,” Ruffalo says. “He had this sad, noble struggle against all the odds and it came at a great cost – a personal sacrifice.”

The film is based on Robert Graysmith's best-selling books, Zodiac and Zodiac Unmasked . Graysmith, a Chronicle editorial cartoonist at the time, worked on trying to crack the case on his own time after Toschi was no longer on it.

But the Toschi-Graysmith connection, Ruffalo says, is crucial.

“[Graysmith] became a way for Toschi to stay involved in the case even when he wasn't a cop anymore,” Ruffalo says. “In a way, Graysmith's work vindicated Toschi's [efforts] – it validated his life.”

In researching his role, Ruffalo also met Bryan Hartnell, who was only 20 when he survived a 1969 Zodiac assault at Lake Berryessa. (Hartnell's girlfriend, Cecilia Ann Shepard, 22, died in the attack.)

“I'd already read so much about the case – I had all the police reports, the pictures, the crime scene investigations,” Ruffalo says. “But to be standing there with person who was actually the victim of this crime? It was intense.”

Hartnell is now a successful lawyer and married with two children. Still, one never really shakes off such an experience, Ruffalo says. “It's heavy to be in that world.”

The media circus that built up around the Zodiac case only added to the weight, he says, and made the killer an unlikely but instant celebrity.

“There was nothing else like the Zodiac at the time,” Ruffalo says. “People had yet to be desensitized to the frenzy and it just had this huge metaphorical significance.”

Ruffalo hopes Zodiac's many dangling threads leave viewers with a sense of the bigger picture.

“Even though Toschi felt in his heart of hearts that Allen was the killer, he couldn't just walk up to him ...,” Ruffalo says. “What does a world look like when you can't solve a crime like this?

“People know they're coming to see a movie about a killer who doesn't get caught. But what gets them is that we live in a world where the guy could still be out there – that's what haunts you.”

© 2007, The Sacramento Bee (Sacramento, Calif.).

Distributed by McClatchy-Tribune Information Services.

Zodiac is currently in theaters.