Glasgow-based David Mackenzie’s gritty prison-set drama Starred Up starring UK hot-shot Jack O’Connell and Australia’s Ben Mendelsohn caused a stir as soon as it premiered in Toronto 2013 – but it’s much more than simply a story of violence and incarceration.

Nearly one year later the drama about an explosive young offender sent to an adult prison inhabited by his father is about to get a US release through Tribeca Film.

Mackenzie talks to Jeremy Kay about the effect of shooting in an actual prison, working with his intense lead men and the father-son tale at the heart of the story.

How did you hear about Jonathan Asser’s screenplay?
It was a spec script by a first-time filmmaker who was a therapist in London and had written a very potent, honest realistic piece which came to us and I responded to it. I felt it was a chance to make a straighter movie than I had normally made – something hard-boiled. I had never done a genre movie before and was excited to have the change to smuggle in a little bit of a heart and soul and softness. It hit me at the right time.

Where and when did you shoot?
We filmed in February and March 2013. The moment I got it I was determined to make the authenticity come alive. We shot in a real former prison. There was very little geographical fakery and the atmosphere of what was going on was imbued in the architecture and almost coming out of the pores of the building and that helped focus everyone’s minds. We had advisors who helped us keep it real. The aspiration was to pump up the tension and inject some humanity and hope in the way the character reaches out and find human connection.

What was the former prison?
We shot at the Crumlin Road Gaol in Belfast known as The Crum. It closed in the late 90s and held political prisoners during The Troubles. It’s a listed Victorian building with a panopticon [central command column] in the middle that looks down on four wings. A lot of modern prisons borrow that same architecture so you can control things. It was the only one we got access to that had not been knocked down. I couldn’t imagine doing this on a set.

Your leads are memorable. Tell us about them
I came across [Jack O’Connell, as the young offender Eric Love] via the normal casting process. He stood out as being the one who could clearly relate to the character and who was interested in being brave enough in taking that character all the way. It’s a fantastic, very strong performance. He is quite well known in the UK from the TV series Skins and has been in quite a few other films, but his time has come now. He’s a fabulous actor. I love Ben [Mendelsohn, who plays Eric’s father Neville Love]. He and Jack come from very different acting and the unpredictability of Ben and the intensity of Jack was a director’s dream.

Their on-screen story is powerful and moving. What were you trying to achieve with it?
It was allowing that father-son relationship, not all of which was fully scripted, to take the journey it went on. Ben can bring vulnerability to a part that doesn’t like to have vulnerability and that was beautiful. The story is to some extent about him becoming weaker and more vulnerable and Jack’s character become more dominant. The ebb and flow is what I was after and the organic process going on within this  narrative that doesn’t have hard lines attached.

Rupert Friend as prison therapist Oliver Baumer faced a particular challenge
Rupert had to ensure he didn’t sail too close to Jonathan [Asser, the screenwriter and an actual prison therapist]. The therapy in the film was Jonathan’s technique, which he pioneered – which was allowing violent prisoners to escalate and de-escalate situation. Rupert did a great job and it was interesting for him to have the real character being near to him the whole time.

You shot in unorthodox fashion. What was that like?
The fact that we shot the film sequentially, allied to the spirit of the actors, made it a very positive experience. I have always wanted to do that. You don’t often get an opportunity to because it’s logistically very hard. It was the idea of going on a journey together and letting the story unfold. I edited it constantly. We had two editors and watched [dailies] every night. At the wrap party we showed the film. It was a very intense process.

What effect did shooting in a prison have on the cast?
You turn up at work every day and half the job is done because the architecture of the place is so hard – it’s steel, stone, concrete and bars and you feel it within every second of what you’re doing. The film is imbued with it. I didn’t use any clapperboards, so there was a sense we were running the whole time and observing, so everybody had to be ‘on’ the whole time. It was an ongoing thing and often they didn’t know when the cameras were running and then they weren’t.