Directed and co-written by Angelina, First They Killed My Father is adapted from Loung Ung's memoir about the Khmer Rouge genocide in Cambodia during the 1970s.

The actress has now revealed her eldest son Maddox, 14, who she adopted from Cambodia, is helping with research on the film, and Pax, 11, is working with the photography department.

"Pax is doing a lot of the stills (photography)," Angelina tells America's People magazine. "The whole movie is from a child's point of view."

Maddox previously worked as a production assistant on his mother latest film, By The Sea, which also featured his adopted dad Brad Pitt.

However, the actress revealed Maddox had to be kept away from the film's set whenever his parents were filming uncomfortable scenes.

"Every time we had a fight scene or sexual scene he obviously wouldn't be near us. So he had to work in the parking lot," she explained.

"But he read the scenes of the day - I didn't even know! One day he came up to me and said, 'Really interesting script, Mom.'"

Angelina and Brad have four other children together - Zahara, Shiloh, and twins Knox and Vivienne.