HOW DO you keep a prison governor happy while also making a television programme that lifts the lid on the endemic drug abuse inside his jail?

Ask Glasgow-based film-makers Jules Kean and Michelle Friel. The duo's particular blend of premium access, relationship building and camera skill has resulted in Girls Behind Bars - a three-part documentary set in HM Prison Cornton Vale, Scotland's only all-female institution. Despite provoking tabloid outrage, the BBC Scotland series has won huge audiences and pleased prison authorities by communicating the hard reality of life under lock and key.

It is all down to hard work, observes Kean: "We immerse ourselves in the situations we are in. We spend an enormous amount of time filming compared with others and come back with a very realistic picture."

Described by its creators as a "straight observational documentary", the programme follows Boys Behind Bars, a similar documentary made by the duo and broadcast in 2006. Girls Behind Bars is certainly eye-opening. In the first episode, viewers watched a young girl trying to commit suicide using a plastic bag, a prostitute attempting to come off heroin and inmates who not only boasted of their easy access to drugs but offered a running commentary on the illegal smuggling as it happened.

BBC Scotland commissioner Ewan Angus - responsible for all TV output produced for audiences in Scotland - says the first episode brought in "the kind of audience we would expect for a football match or soap opera". Angus says 33% of the entire TV audience in Scotland had tuned in when the show began on Wednesday, October 1 at 10.45pm, a figure which grew to 44% as it progressed; the episode broadcast last week attracted 39%, around 428,000 people. The third and final episode airs on Wednesday.

But not everyone likes it. The Daily Record's TV critic wrote of "living rooms across the country wincing" at the film's "moral dubiety" while questioning Friel and Kean's motives and implying they were complicit in the inmates' drug use.

Kean, 40, started out making commercial videos for private companies but went on to direct Bafta-winning series Chancers about young offenders in 2002. Friel, 32, cut her directing teeth making documentary programmes for BBC Scotland such as the Tales From The Edge series. Both are "fiercely proud" of their labour-intensive techniques.

While some film-makers try to shoot as much footage in as short a time as possible - Kean calls them "run-in run- out monkeys" - Friel and Kean take the opposite approach. Working from 9am to 9.30pm and staying in a B&B beside the Stirling prison, they established their own relationships with the women over long periods, producing 250 days' worth of footage for only three hours of finished film. That doesn't count the time the pair spent working solo inside the prison, winning the women's trust.

Although both deny that they ever pretended they were the girls' best friends, Friel admits to giving her mobile number out to individuals featured in her films, and Kean says he has tried to help one of the people featured in Boys Behind Bars find work on release.

Kean is no stranger to controversy. Although Chancers won a Bafta in 2002, it also led to the rehabilitation scheme it featured - the Airborne Initiative - being shut down by the then Scottish Executive which was said to be unhappy with the misbehaviour they saw on screen. Although officially the film's director, TV sources say Kean had very little control over the final edit, despite shooting much of the footage. The Airborne affair still rankles with Kean who refuses to discuss it. "I'd just sound negative," he says.

Kean and Friel formed their own company in 2005 to maintain complete creative control of their output, ensuring that the promises they make to subjects or sources are kept. The approach has paid dividends. Boys Behind Bars, the duo's last BBC Scotland film, shot inside Polmont Young Offenders Institution, won a 2006 Royal Television Society award and led to the Cornton Vale commission.

Friel says spending the time with the women is the only way to get the desired results: "I was in the suicide-watch section of the prison for three months, which houses the most damaged women. It is not an easy environment to get people to open up in."

Adds Kean: "We work very hard, sometimes following a character for days and then nothing happens with their story. We have to be there when something happens. We would never encourage anyone to do anything for the cameras."

Both film-makers say they explain the process in detail to their subjects. Kean says that when somebody starts to do something illegal on film they warn them of the implications. Both say they will not inform the authorities of any illegal action apart from threats of violence.

Ian Gunn, governor of Cornton Vale, praises the series and says the pair worked closely with him to ensure there were no surprises when it hit the TV screens. Gunn was offered a preview screening of the film and believed if he had requested any changes to be made then the film-makers would have complied.

At times the film-makers took direction from the staff: for example, when Gunn felt that a suicidal girl featured in the first episode was playing up to the cameras he asked the pair to stop filming, which they did. But the film-makers insist that creative control over the finished film rested with them.

Gunn applauds the techniques the film-makers used to get the girls to open up about drugs, even though footage revealed how easy it is for the inmates to run rings around the prison staff. "It was a skilled technique the film-makers used and they worked hard to get the confidence of the prisoners. We don't condone drug taking but we have huge difficulties in policing it."

"I suspect that the people who decided to talk to the cameras didn't fully appreciate that what they were saying was going to be broadcast in a different context and there are a few people thinking, oops - I wish I hadn't said that'."

So after focusing so heavily on prisons for the first three years of their career, what projects are firing Friel and Kean's imaginations now? Kean smiles wolfishly: "We'd love to do something on newspapers, and it would have to be The Sun."

Now, how to keep the editor of The Sun happy while also making gripping TV is another question entirely.