Greenwald's feature and TV films, including "Steal This Movie," a biopic on Abbie Hoffman, and "How to Murder a Millionaire," were hardly classics. His fascination with documentaries was born in 2001 when he helped friends produce a film on voting irregularities in Florida that clouded the election of President George W. Bush. That was followed by "Uncovered: The War on Iraq," which documented the Bush administration's misleading the U.S. into a war that cost an estimated $2 trillion and damaged America's international stature.

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"People kept coming up to me and thanking me," he said, recalling the line that stretched around the block at the premiere in Santa Monica. "In my previous life, people had liked the films or hated the films and everything in-between. But the notion to be thanked for doing a film, and that it was impacting people in this way, was heroin.... That is when I said this [documentaries] is what I want to do."

He laughed: "You don't have stars to get out of the motor home. You don't have hair and makeup," he said. "We don't have a budget, so we can't buy our way into awareness." He later added: "We don't get to certain people, but that's more about distribution than tone.... I'm a neurotic Jew from New York. I could do five hours on where we've failed."

Making movies with Greenwald "is exciting," said Caty Borum Chattoo, a documentary filmmaker and a producer on his 2005 "Walmart: The High Price of Low Cost." "It feels as if you're advocating. A lot of filmmakers have picked up on this model.... It's about luck, opportunity and picking up on the social zeitgeist."

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"Koch Brothers Exposed" called Charles and David Koch, whose interests include oil, chemical and wood processing corporations, the "poster boys" for the 1% of wealthiest Americans. It portrayed the brothers' conservatism and political contributions as threats to voting rights, public schools and environmental regulations.

"Robert Greenwald continues to be dishonest about Koch," Mark Holden, general counsel, Koch Industries Inc., posted on the corporation's website. He accused Greenwald and his team of "harassing behavior" and said the producer's video attacks are "outright smears" that consist "of the derivative rehashing of distortions and fabrications made by far-left bloggers."

The director is in constant search of injustices. To coincide with the release of his drone film last year, he arranged for visas for a Pakistani village family to travel to the U.S. and testify before Congress — only a handful of lawmakers attended — about the missile that killed their grandmother.

"I no longer love blue skies," Zubair Rehman, 13, who was picking okra with his grandmother on the day she died, testified through an interpreter. "In fact, I prefer gray skies. The drones cannot fly when the skies are gray."

"The thing that keeps me sleepless many nights is the sense of responsibility," said Greenwald, who plans to show the film on 150 college campuses this year. "It's literally people's lives … and people are investing their beliefs in your ability to tell a story that will make a difference."