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Both Banksy and the makers of “Catfish” insist their films are completely true.

But then there was the public furor over “I’m Still Here,” the documentary capturing actor Joaquin Phoenix’s late-00s meltdown — most notoriously in an appearance on the “Late Show” — and his abandonment of acting for a rap music career.

Director Casey Affleck announced shortly after the film opened in theaters that, contrary to his previous claims of authenticity, the whole movie was a planned stunt, including Mr. Phoenix’s bizarre public behavior.

Though documentaries still only account for a fraction of the Hollywood-dominated U.S. box office, the genre’s clout has rarely been higher.

Hundreds of nonfiction films come and go unnoticed, but a few documentaries such as the global-warming manifesto “An Inconvenient Truth” and last year’s education-reform call to arms, “Waiting for ‘Superman,’” have changed the public debate on their subjects. For example, President Obama invited the latter film’s child stars to the White House last year.

And the fusion of fact and fiction is occurring on the other side of the divide as some of the documentary genre’s stylistic tropes are showing up in — of all places — horror films. Such films as “The Blair Witch Project,” “Paranormal Activity” and “The Last Exorcism” all employ documentary elements to heighten their scares.

Those who don’t like the new, refashioned documentary format better get used to it. Ms. Gilden Seavey said she routinely has to push her documentary film students to examine whether something is “authentic or not” in class discussions.

“They have a lot less discomfort with things that are not true, or things that are sort of true, and it concerns me,” she said. “They have been influenced by reality television. … They’re willing to live in the gray areas of the truth.”

Jerry Misner, director of the new documentary on illegal immigration, “Southern Exposure,” agrees reality television is leaving a mark on the minds of tomorrow’s filmmakers.

“Take any reality show on right now. The objective isn’t enlightenment, it’s entertainment, pure and simple,” Mr. Misner said.

Tom White, the editor of Documentary magazine, sees these discussions as “healthy” for the genre, but adds documentary filmmakers should still behave in a responsible fashion.

Mr. White points to the Documentary Filmmakers’ Statement of Best Practices in Fair Use, a handbook of professional ethics drawn up at American University as an example of the industry policing itself.

“It’s like a physician — at first, do no harm. Respect your subjects who have agreed to be a part of this,” Mr. White said.

He adds that while some film critics questioned the validity of “Catfish,” others raised their eyebrows over the film’s treatment of one character, who appears to be suffering from emotional issues, and questioned whether the filmmakers’ public exposure of that served her best interests.

But defenders of not strictly literal understanding of “truth” include some of the genre’s greatest artists.

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