What does it take to catch a killer? For Robert Graysmith, author of “Zodiac” and “Zodiac Unmasked: The Identity of America’s Most Exclusive Serial Killer,” the answer to that question is simple: obsession. An obsession that would take priority over his family, his job, and his safety.

“I am simply stupid,” Graysmith says, referring to his lack of prudence when dealing with the Zodiac Killer.

But Graysmith was not a police inspector or a crime reporter like the other three men focused on the case; he was a cartoonist for the San Francisco Chronicle who liked going to the library and solving encrypted messages. When the threats became more aggressive, he learned to take, well, minor precautions.

“I would open my door with my foot because you know, it gets to you a little bit,” he recounts.

“Zodiac,” a new film directed by David Fincher of “Se7en” and “Fight Club” fame, features Academy Award nominee Jake Gyllenhaal in the role of Graysmith.

The film chronicles the period between 1969 and 1991, in which the Zodiac Killer murdered an unknown number of men and women in southern California, taunting the police and public with encrypted messages of past murders and future “aspirations.” During this time, four men—Graysmith included—dedicated their lives to the task of solving the Zodiac mystery.

“I was on [the case] so early,” says Graysmith, “talk about obsession.” However, he claims that he’s “much better now.”

Self-described as “shy and deferential,” Graysmith says that no one expected him to become so absorbed in the ordeal.

Today, 28 years after the first murder, Graysmith still seems like a pretty normal guy. Perhaps a bit eccentric when it comes to his crime novels—seven of which have been published, three of which have been made into films—he is enthusiastic about what he does, and happy to let anyone with the slightest inclination know the thoughts in his head.

According to Graymsith, Gyllenhaal, though riveted by the script, hesitated to take the role, afraid to open a Pandora’s Box and reveal the psyche of a man so in vested in a killer. However, after meeting with Graysmith to better understand the character he might portray, Gyllenhaal accepted the role.

“Jake reached in,” notes Graysmith, recalling his many one-on-one sessions with the star.

Besides advising the actor, Graysmith worked with Fincher to make the film as accurate as possible. The director recreated everything from the wiring of the ceilings to the trailer of the main suspect, and even the clothes on Gyllenhaal’s back.

“I loaned most of my stuff…the code books were my actual books,” Graysmith recalls with enthusiasm. “Jake was wearing my clothes. Those were the clothes I wore!”