Angelina Jolie thinks making a movie in Cambodia helped her oldest son ''grow and evolve'' in a way she couldn't help him with.

The actress-turned-director enlisted 16-year-old Maddox to serve as executive producer on 'First They Killed My Father' - which is based on author Lung Ung's account of the Khmer Rouge 1975-79 regime - and she was ''so proud'' to see the teenager working hard in his home country, from where she adopted him as a baby.

Speaking to Psychologies magazine, Angelina - whose 13-year-old son Pax was a stills photographer on the set - said: ''It was a joy. I wanted them to understand their world and be able to go out and live and make friends. Yes, we were working on something important and I wanted them to have a strong work ethic, but I was also so proud when I looked over and saw Mad and Pax working with the crew and creating something. It was a natural process and I think it helped Mad to grow and evolve in a way that I couldn't do for him. His country had to teach him. He was very involved in the production and Pax worked as a set photographer.''

The 42-year-old star - who also raises Zahara, 12, Shiloh, 11, and nine-year-old twins Knox and Vivienne with estranged husband Brad Pitt - had known for many years that she wanted to make the book into a movie with her son's involvement, and was very ''impressed'' with the work he put into the project.

She said: ''He earned that [executive producer] credit. We'd had the script for a few years and Mad has known of Loung's story his whole life. I told him, 'One day, son, you'll be ready and you'll tell me when it's time to go deeply into your country. Bt I need your help. You have to do it with me.'

''And because the story is from a child's point of view, it was good to have somebody younger like Mad there to say, 'You're losing my attention'. He really impressed me.''