WHEN the new head of MI5 was revealed last week to be Andrew Parker, the organisation's current deputy, it still seemed remarkable to see a photograph of the man and learn, among other things, that he was a keen ornithologist.

It was not so long ago that the very existence of the secret services was itself a secret.

Contrast this with Israel, where six former heads of Shin Bet, the country's domestic secret service, talk about their work in The Gatekeepers, a new documentary by Dror Moreh. Nominated for an Oscar, Moreh's film contains so many startling insights that after a while it becomes easier just to leave your jaw on the floor. How on earth, I ask Moreh, did he get them to speak? If I called up MI5 and asked to interview six former heads about state security, I tell him, I would get short shrift.

"First of all, let me ask you, did you try?" The reply "Er, no," is greeted with a blast of laughter.

To be fair, Moreh, 51, had to do a bit more than place a call to end up with a film as remarkable as The Gatekeepers. The tale begins with another documentary he made, this one about former Prime Minister Ariel Sharon. Moreh wanted to know what had made such a hardline premier withdraw Israeli troops and settlers from Gaza.

He was told Sharon had read a newspaper article quoting former heads of Shin Bet warning that failure to disengage from Gaza would lead Israel towards catastrophe. For Moreh, this was a "lightbulb" moment. If he could get the heads of Shin Bet to speak on camera then who knows, further momentous events might happen.

He thought long and hard about the best way to penetrate such a disparate circle of men. "They don't have a club where they drink and smoke cigars. Actually, they don't like each other so much," he laughs.

He decided his first contact would be Ami Ayalon, the war hero appointed after Yitzhak Rabin's assassination. It took a month to get a meeting, and after a long session Ayalon agreed. From there, Moreh was put in contact with the others. "They interviewed me in the beginning. And believe me, they know how to interview."

Before the camera was switched on Moreh scaled a mountain of research and had more conversations with his subjects. In total, he ended up with 10-15 hours of filmed interviews with each man. Key to the success of the interviews was Moreh's ability to know when to prompt, and when to keep quiet.

"At numerous times I bit my tongue or my jaw dropped while listening to them. They have amazing stories to tell, and amazing life stories to share."

Moreh found himself not just amazed but surprised at the men themselves. He went in with notions of what they might be like, only to find, human to human, something different. "Bear in mind those people were responsible for life and death decisions, all their careers were life and death decision-making."

In making The Gatekeepers, Moreh was influenced by Errol Morris's documentary The Fog of War. Just as Morris's interview with Robert McNamara, former US Secretary of Defence under Kennedy and Johnson, was the ultimate insider's account, so Moreh wanted a similar take on Israeli history in his film.

The Fog of War, dealing as it did with Vietnam and other matters, had a lot of archive footage to draw on. For his film, Moreh was faced with the problem of how to illustrate events that were top secret at the time. "I knew from the beginning that I would have a very tough time telling the stories they tell visually. There were no cameramen there to document what they did, and if there were they would not give me that kind of [material]."

Moreh, a cinematographer before he turned director, needed to use new tools in new ways. Working from photographs and other material, he reconstructed events using computer-generated imagery. "I wanted it to be as close as it could to how it looked in reality." He also wanted situations to appear as they did to the Shin Bet chiefs back in headquarters taking the decisions.

The reaction in Israel to the film was "overwhelming", says Moreh, and remains so. From opening in just two art-house cinemas it ended up in multiplexes that had never previously shown a documentary.

"It's still running in Israel, 70,000 people have watched it. It's the best-grossing documentary in the history of Israel."

Following its release in the US, Moreh, who lives in Tel Aviv, had a lot of job offers from America. He is now working on a five-part series on The Gatekeepers, and a book, but he is keen to get back to where he started in cinema, in drama. As he says, there's a lot of material about. "The secret service, clandestine operations, targeted assassinations, drone attacks and politics."

Speaking to the security chiefs, and hearing about opportunities missed, made Moreh more pessimistic about the future, and more cynical about political leaders. But he still hopes the film will open the door to progress.

"No-one understands the conflict between Israel and the Palestinians better than these six men. When they speak, leaders listen. Perhaps the time has come for the gatekeepers to address the people at large, and not just the inner circles of decision-makers."

The Gatekeepers, Cameo, Edinburgh, from April 12; Filmhouse, Edinburgh, April 26-May 2; Cameo, Edinburgh and Belmont, Aberdeen, May 7.