First They Killed My Father Author Says the Kids Looked up to Maddox Jolie-Pitt as a ‘Big Brother’ on Set

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Following the Cambodian premiere of Angelina Jolie’s new film First They Killed My Father in Siem Reap, Cambodia Saturday, co-screenwriter, rights activist and author of the book that inspired the movie, Loung Ung, spoke to PEOPLE about what it was like working on set, the challenges of portraying the Cambodian genocide on film and eating tarantulas to lighten the mood among the cast and crew.

Ung, who has been friends with Jolie since First They Killed My Father was published in 2001, said that there was a real “family feel” on set, with Jolie’s son Maddox, 15, helping with production and her second eldest Pax, 11, shooting stills.

“I have watched the young people grow into really kind, empathetic, curious human beings,” Ung tells PEOPLE. “Pax is a talented photographer and Mad is so kind and generous. The cast kids, they all looked up to him as a big brother.”

Ung says she is “so proud” of Jolie’s sons, who would play with the young kids on set and “make them laugh.”

“There were sad days on the set and there were difficult long hours. And to do it in an environment where people love and respect each other was exactly what we needed to get it done,” she says.

The movie is based on Ung’s autobiography of the same name and tells the true story of her experiences during the Cambodian genocide at the hands of the Khmer Rouge communists during the 1970s. An estimated 1.7 million people died during the Khmer Rouge’s disastrous campaign to turn the country into an agrarian utopia.

Ung was just 5 when communist soldiers seized her hometown, the capital Phnom Penh, and emptied the city of its inhabitants. Thousands of intellectuals and members of the middle class were executed in a purge against what the Khmer Rouge considered was an influence from the West. Those who were not killed were forced to work in labor camps where they suffered from disease, malnutrition and abuse. Ung, whose father, mother and two sisters died during the horrors, says that her surviving siblings “love and respected Angelina so much” that they were able to trust her to tell their story.

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“To have this beautiful film about family, to have this beautiful mother doing a film about family, I had complete trust that our story was in good hands. And my siblings felt exactly the same way. We are so honored and proud that this wonderful mother…