Anyone going to see a movie that’s supposedly “based on true events” knows that what they’re actually getting is the Hollywood version of events. That means certain elements or characters have been cut, the timeline has been reordered, and things have generally been made more dramatic to serve the narrative. But for the 2007 film Zodiac, David Fincher and his team wanted to make sure that certain aspects of the true crime story were as close to the reality as possible. Unsurprisingly, those are some of the scariest moments in the film.

“What they’ve captured on the film that you see when Cecilia is being stabbed, that’s the flash I saw happening,” Bryan Hartnell tells Film Radar. Hartnell was a survivor of an attack from the Zodiac Killer at Lake Berryessa in September 1969. After being stabbed six times in the back he witnessed the murder of his friend Cecilia Shepard. That attack is one of the more terrifying moments of the film thanks to the filmmaker’s acute attention to detail. From the Zodiac’s creepy executioner hood to the idiosyncratic feeling of his conversation with Hartnell, everything is as close to how it actually happened as possible. “It’s an eerie reproduction of what happened in my vision,” Hartnell adds. “I couldn’t have scripted it better.”

According to Film Radar, giving Zodiac this visceral, realistic feeling took a lot of work. The people behind the film were given access to case files, interviewed survivors, and practically built their own investigation into the Zodiac Killer’s identity simply to make the finished product feel like the real-life story they were trying to tell. Considering Zodiac is one of the best true crime thrillers in film history, it’s safe to say the effort was worth it.

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