As a tween in Melbourne, Kitty Green believed in the myth of the perfect American family. A steady diet of The Brady Bunch and Full House will do that.

But that was before 6-year-old beauty pageant princess JonBenet Ramsey was found slain in 1996 in Boulder, Colorado.

Green, now a film-maker, had mistaken fiction for reality. Decades later, she's examining that elusive line in the inventive documentary Casting JonBenet.

The film doesn't re-investigate the case. Instead, Green travelled to Ramsey's hometown to learn about the memories and conspiracy theories of the people who live there. She wasn't interested in the truth of the case so much as deeper truths about human nature - and she made her movie using elements from traditionally fictional films.

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She had actors in costumes re-enact unverifiable episodes on sets that looked like the Ramseys' house. The movie doesn't even contain archival footage.

Some moviegoers expect documentaries to deliver straight, unadulterated facts. But that's not what viewers get from acclaimed documentarians such as Green, Joshua Oppenheimer (The Act of Killing), Sarah Polley (Stories We Tell) and Robert Greene (Kate Plays Christine), who add dashes of performance to their versions of reality.

Documentaries have always straddled the line between art and science, according to film scholar Judy Hoffman, who teaches at the University of Chicago.

Eadweard Muybridge's galloping horses gave the world irrefutable facts. His footage showed what was previously unproved: When horses run, all four hoofs come off the ground at once. But some of the earliest examples of documentary - before the word existed - were experimental and surreal. Hoffman points to the 1929 Russian film Man With a Movie Camera, with its avant-garde cinematography and editing, and The Song of Ceylon, from 1934, which added sound effects and montages to its chronicle of life in Sri Lanka.

"People have always had the idea that documentaries are independent and they have this morally superior purity to them, but that's never been the case," Hoffman said. "Every film is a construction."

Casting JonBenet just tends to show its construction more conspicuously than others. In it, Green put out a casting call for local actors to play the parts of the people involved in the Ramsey case. Little girls with blond ringlets showed up to play JonBenet, and older residents portrayed the girl's parents, the police chief and a creepy ex-con who falsely confessed to the slaying.

Hannah Cagwin in a scene from,
Hannah Cagwin in a scene from, "Casting JonBenet." Photo / Netflix via AP

Part of the movie consists of dramatisations of what might have happened that night, but most of it is interviews with the locals, who relay their memories of the case and their speculation about the unsolved crime.

They reveal intimate anecdotes about themselves - their cancer diagnosis, their murdered family member, the abuse they suffered as children. The message is that each person's past informs their judgment.

"Immediately it became personal," Green remembers of the interviews she conducted with about 200 people. "'My mother has bipolar disorder, so I think it was the mother' or, 'My brother used to hit me as a kid, so I think it was the brother', and then they would tell me their own stories ... and I'd be captivated."

In Kate Plays Christine, Robert Greene followed actress Kate Lyn Sheil as she prepared for the role of Christine Chubbuck, a newscaster who shot herself on live television in 1974.

Greene says that making a documentary about an actress naturally makes audiences question if what they're seeing is real. But he felt as if he was being more open with his viewers than a lot of film-makers are. He was holding up his manipulations for the world to see.

"I think authenticity is a thing we're selling on the side of the road like a peddler with watches in his coat - and I believe that's a fallacy," he said. "But I still believe there's value in observing reality with a camera and there's value in creating something out of those observations."

There's something seductive about thinking that, when we watch a documentary, we're getting nothing but the truth. But any savvy consumer of journalism knows that objectivity is a myth. Storytellers are constantly weighing which anecdotes to include and what to leave out. These documentarians go a step further by inventing a performance that augments reality. By doing so, they remind us how thin the line between fact and fiction can be.