Robert H Lieberman is a filmmaker, novelist, physicist and one time vet school drop-out. Born to Jewish parents in New York shortly after his family fled Nazi Germany, the 74-year-old says he has somehow always found himself in interesting places at important moments history. His documentary They Call it Myanmar – shot clandestinely over two one-year stints – won global plaudits when released in 2012, and featured an interview with Aung San Suu Kyi shortly after her release from house arrest. Now Lieberman has turned his sights to Cambodia, and has just completed filming for a documentary on the Kingdom: Breaking Baksbat (‘broken courage’ in Khmer).

Before flying out of Phnom Penh this weekend, Lieberman spoke to Harriet Fitch Little about the new film’s outlook, and detailed how exactly he secured an interview with Prime Minister Hun Sen .

You’ve made half a dozen films and written half a dozen novels all while teaching full time in the physics department at Cornell University. How does that work?

Cornell’s very generous. They know what I’m doing and cut me some slack – this term I’ll be coming back a bit late. The good part is that physics and engineering make you very analytical. You can drop me naked into any country in the world and I will survive. The bad part, and it’s only come recently, is that it becomes highly stressful – I’ll be working on a book at home, or a film, and I’m exhausted, then the kids start asking me physics problems and I have to totally switch gears. I was better when I was younger. I’m going to be 75 soon, and I’m still doing it. Why? I love it. I plan to die at the blackboard. I’ve told my kids that. And I already have my epitaph.

It's ‘He wrote different books and he made different movies’. In case I die and I don’t get a chance, would you request my wife not to forget that?

Why did you want to make a film about Cambodia?

I’ve got to tell you my history, then you’ll understand. My parents escaped Hitler with my older brother, and I was born in the US. My father lost his mother, his sister, her husband . . . he tried to get them out and couldn’t, and for the rest of his life was burdened with tremendous guilt. It had an effect on me even though I wasn’t there: my bags are always packed, everything is temporary, I thought I’d be dead at 40 . . . I just had this sense of mortality. Trauma is passed on psychologically and there’s something called epigenetics that they’re studying now which suggests that it might also be passed on genetically by changes in the DNA sequence. So I’m connected to Cambodia in that terrible sense. Being a child of the Holocaust, I think I understand what is potentially going on here.

Robert H Lieberman has just finished shooting his documentary about Cambodia, Break Baksbat. Victoria Mørck Madsen

So this is a film about the Khmer Rouge?

This is not a Khmer Rouge movie. But to know the present you have to know the past, so we deal with people who have experienced this trauma and then we move on through successive generations. My focus has shifted enormously since I started making the film, especially now I’ve had such positive experiences with the 20-somethings here. They’re smart, energetic... we’re ending the film on a real high note.

You’ve interviewed several prominent politicians for Breaking Baksbat. How political is the film?

Yes, I’ve got Hun Sen in the film, also Sam Rainsy, his wife [Tioulong Saumura], Son Soubert . . . and I do have filming of the demonstrations and their subsequent put down. But it’s not a political film. It’s not that I’m afraid of the politics, but including politics is going to date it, and I want to make movies that last. It’s really a film about mentalities. I want to know what’s going on in people’s heads.

Who did you most enjoy talking to?

John Gunther Dean [US Ambassador to Cambodia in 1975]. We got in touch through a friend and it turns out his parents were in Breslau and fled to the US shortly after mine, to Kew Gardens [in New York], which is where I grew up. I asked what school he went to, and it was my school. And it went on and on, the similarities. He’s 90 years old now and he left Cambodia with huge regrets. He talks about being in his office the day before he left. He said he was crying, because he knew he was leaving Cambodia to butchers. He tried to negotiate a settlement but Kissinger wouldn’t let him.

In the rough cut I’ve seen of the film, people speak frequently of Cambodia being at a “tipping point”. Why now precisely?

There are lots of people in the film saying that. I think it’s that you have this demographic bulge of people born after the Khmer Rouge, and they are looking outside of the country in a way they never did before. So either the politicians are going to have to change or they’ll be bypassed. That’s why this is the tipping point.

Do you agree with that analysis?

I was in Czechoslovakia the year the [Berlin] wall fell. I was in Hungary that year as well, and I was in Romania when [Nicolae] Ceausescu had just been assassinated. Everywhere I go there is change. Everyone is convinced that I have created these things – that I have a string I pull and things happen. I don’t know if that’s true or not, but I have always been at turning points and I sense that this is a turning point for Cambodia.

Interview edited for length and clarity.

A date with PM Hun Sen

Robert H Lieberman recounts how he secured a rare interview with Hun Sen via a letter of introduction from ex-US ambassador to Cambodia John Gunther Dean. After a lengthy back-and-forth, the filmmaker met the Cambodian prime minister in his New York hotel suite during a state visit. Lieberman puts his success down to submitting talking points in advance (“very easy questions – things I’m not really interested in”) and then, once face to face with Hun Sen, asking the prime minister if they could “throw them away”.

According to Lieberman, what followed was a surprisingly candid two hours. “I felt there were moments where he was actually asking for sympathy and understanding, and that was real – two old farts eyeball to eyeball,” he said.

Lieberman said he challenged Hun Sen on sensitive issues including the state’s violent suppression of protests, (Hun Sen’s retort, paraphrased: “You have shootings here in America by the police. Is that Obama’s fault?”), but that there was no discussion of the CNRP: “I said ‘tell me about Sam Rainsy’ and he said ‘go talk to Sam Rainsy’.”

Lieberman said that his favourite moment came when Hun Sen was expounding on his nonviolent disposition. “He said to me something to the effect of, if you were going to have dinner at my place and you were relying on me, you would go hungry. I couldn’t even kill a chicken,” Lieberman recalled. “I’ll never forget that. I can’t say the same for myself – I’ve killed a chicken. And a duck.”