Documentary maker Alex Gibney on Mea Maxima Culpa

His new film, Mea Maxima Culpa: Silence in the House of God, investigates Father Lawrence Murphy, a priest accused of abusing 200 boys at St John's School for the Deaf in Milwaukee, Wisconsin, between 1950 and 1974. Murphy died in 1998.

Though the particular case is historical, Gibney's film uses it to show generally why the issue of abuse of power within the Catholic Church has not gone away, and why cases will continue to surface.

Given the amount of coverage the topic has received, Gibney did wonder at the time what else he could bring to the subject. "I asked myself that question a lot," he says when we speak at the London Film Festival.

Two things persuaded him that this was a film that had to be made. First, he felt no-one had yet taken this kind of story and followed it all the way up the chain of command to the top – the Vatican. Second, if it ever comes time to write a history of clerical abuse, the victims in the Milwaukee case will have a special place.

"I'd seen a lot of films about victims which were very heart wrenching. This was a film where survivors fought back, and they were heroes. That was very meaningful to me, very emotionally powerful. Because of who they were, deaf men, it added a special kind of metaphorical and cinematic resonance to the whole theme, which is the story of secrecy and silence."

However hidden or complex the subject, be it torture in his Oscar-winning Taxi to the Dark Side, or financial manoeuvrings in Enron: The Smartest Guys in the Room, Gibney's films abide by the basic journalistic rule of seeking responses to questions raised. In Mea Maxima Culpa, which went on to win the London Film Festival's best documentary award, that meant knocking on the Vatican's door, but to no avail.

"I kept knocking, and all I would hear was the echo inside. I didn't hear anybody rushing to the door to open it."

That was not reason enough to back off, however. "It is very hard to take no for an answer, but the fact is I've learned there is always a way to tell the story. You just do the very best you can." In Mea Maxima Culpa, for example, he secures other, highly significant, interviewees.

When financing a documentary, Gibney has found it can be risky to raise money on the basis of access you might get. Early on in his career, for instance, he was promised funding for a film if he could get into North Korea. As an American, he never did.

"It's better, and often ends up getting you more access ironically, if you just say, 'I'm going to make the film, you've got to trust me, that I'm going to get something.'"

When he made Client 9: The Rise and Fall of Eliot Spitzer, Gibney started the film about the former New York governor brought down by a prostitution scandal with no guarantee his subject would agree to talk.

"In a way that ended up being rather effective because I went and told him directly. I said, 'I'm going to make this film whether you participate or not, so I hope you do.'" He did.

Research and preparation over, every documentary comes down to the interviewer's ability to get the right people to talk. "I try to do everything in my power to persuade them, but there are limits," says Gibney. "I'm not going to pay somebody a lot of money to talk to me." It's one thing to pay for the use of photographs, he says, but quite another to pay for interviews.

"I don't like it. Some people do and don't have a problem with it. I suppose if you do you can just disclose it. But I think it does distort the process.

"It is all about the disclosure at the end of the day. Sometimes I've been in situations where people have said I'll give you access if I can have editorial control. That's not a deal I can make."

Gibney was born in New York into a journalist family: his father, Frank, edited Newsweek and Life magazines. A graduate of Yale and UCLA film school, Gibney has now been in documentaries, writing, directing, and producing them, for more than three decades. While the 59-year-old might be regarded by some as the gold standard, he is not one to issue golden rules about the craft.

"I don't like golden rules, generally speaking. One of the refreshing things about the documentary today is that there are so few golden rules. Documentaries used to be full of golden rules and I think they suffered for a long time because of them. The only golden rule I try to keep in mind is that I serve the viewer, and my job is to try to make films that engage them."

I ask what was the toughest film to make, the one that had him breathing a sigh of relief when it wrapped. The answer fires back before the question is finished: "Taxi to the Dark Side." Gibney's 2007 exploration of what happened to prisoners of the US in Guantanamo and Iraq involved harrowing research.

"It actually changed my personality while I was working on it because looking at those grisly images was hard. It was hard to get people to talk and it took a long time, and hard to find materials.

"Also, personally for me it was very tough because it was something that my father really wanted me to do. He had been a Navy interrogator in the Second World War. He also died during the making of the film. I shot him when he was very ill and put a piece of him in the movie.

"For all those reasons, what's that Janis Joplin song, Piece of My Heart? There's a piece of my heart in that dark, dark film. It changed me in some fundamental way."

How? "It made me see things about myself and about human beings that were hard to bear. How brutal we can all be, and yet we live in a world where it's not so simple as to say that person is a bad person, this person is a good person.

"Under the right circumstances all of us can do appalling things."

His forthcoming films include a documentary on the disgraced cyclist Lance Armstrong, and We Steal Secrets: The Story of WikiLeaks. When we speak, he has not been into the Ecuadorian embassy in London where the organisation's founder, Julian Assange, is living after being granted political asylum.

Gibney has, however, had "a lot of interesting contact with the WikiLeaks folk".

When Gibney finds something interesting, headlines usually follow.

Mea Maxima Culpa, Glasgow Film Theatre, March 29-31; Filmhouse, Edinburgh, April 2-4.