EVERYONE is looking for Gary but no one has seen him. His sister Laura says she spoke to him a while ago, but thinks he might be living in the local hostel. Darren Hercher, the film-maker, also arranged to meet Gary a few days ago to talk about a new documentary he’s just made that features the teenager. But Gary didn’t show up. This is not unusual: Gary often doesn’t show up.

The reasons for the uncertainty in Gary’s life are clear in Hercher’s documentary. Both Gary’s parents were heroin addicts. He was taken into care when he was five. And, just as he emerged from care, his mother died of cancer. Then Gary had nowhere to live so he set up camp under a tree in the woods near the old Sighthill tower blocks. The last shot in Hercher’s film is the teenager, homeless, and sitting round a fire in October, living wild in Scotland’s biggest city.

I’m meeting Hercher in the Sighthill estate in Glasgow to talk about the film and he tells me about the first time he met Gary. The 19-year-old looked very aggressive, says Hercher. He was wearing combat trousers and an ear-ring that said “f*** you” and he was the kind of teenager most people would be afraid to speak to. Indeed, when Hercher introduced himself, the first thing Gary said was “nobody ever speaks to me”. But Hercher did and he could see that under Gary’s scary exterior, there was a lovely, caring guy.

Hercher says he also knew in that first meeting that Gary was exactly the kind of person he was looking for to feature in his film, the second that Hercher has made about Sighthill. As well as Gary, the new film features Robert, an 11-year-old living in a big family of girls, and Yonan, a refugee from Iran. Hercher, who’s 39, says he could have made a film about the big 1960s Sighthill blocks being pulled down and the residents being moved into the new houses nearby – and that does feature in the film. But what he really wanted to do was what he’s done in his other films: go in close on relationships, and people, and their pain.

Gary’s pain is obvious from the start. “I’m a baby of two heroin addicts,” he says. “I’m sitting here waiting on my family, waiting on my mum, to stop taking heroin so I could get back home, but they never.” There’s also a striking sequence in which Gary returns from his mother’s funeral, raw and angry, and smashes the first thing he can find, a mirror. And he has few expectations about his own future and the prospects of him ever leaving Sighthill and living a more conventional life. “I’ll be here for the rest of my life,” he says.

Hercher doesn’t hide any of these details in his film, or the fact at Gary is addicted to legal highs, but it’s also clear there’s potential in the young man. “Gary has spent a lifetime in care,” says Hercher. “It’s very, very bleak story to the point where I didn’t want to go into too much detail in the film. It was very dysfunctional and family and relationships and a settled home life were unfamiliar territory for him.”

But what was interesting for Hercher was the way Gary and his friends created their own family unit - and this fact, rather than the physical regeneration of Sighthill, was the key for Hercher. A new house is not going to magically fix what is not working in someone’s life, says Hercher – it’s all about relationships. “If you want to juxtapose Gary and the other boy in the film, Robert, that is the main difference,” he says. “Robert is at home; his parents love him. It’s a big family and all the rest of it, but it’s a loving family and they’re together and that’s the difference.”

You can see from Hercher’s own life why he might have come to this conclusion about the importance of relationships. He grew up in the Hercher family, fairground travellers that regularly tour the north of Scotland, and it was not always an easy childhood. Because of the regular travelling, there are gaps in his education; his parents also split up when he was a teenager and a rift between his father and his uncle has caused tension and trouble among his relatives.

Hercher also believes he was never really suited to the fairground life. “I always was a bit of a dreamer as a kid so I wasn’t the best fairground apprentice because my head was always somewhere else. I was into sketching things, documenting stuff, even at that age – sketching the rides and the people and all the rest of it. So I think at that young age I probably knew that I would end up doing something else.”

In the end, it turned out Hercher didn’t really have a choice as, after his parents separated, he and his mother left the fairground to live a more conventional life in a house in Invergordon. After school, Hercher then moved to Wales to study photography and, after a stint coaching soccer in the US, worked as a freelance cameraman in London.

Throughout it all, his relationship with his father was tricky and for ten years, they barely spoke. Then, in 2007, Hercher made The Downhill Racer, which focused on his life in the fair, but was essentially an attempt to reconnect with his dad. “He was a bit of a drinker so we kind of lost him for a few years so it was me trying to rebuild relationships so I did that, I tried to do that by making a film about it.” Hercher also likes to maintain a relationship with his uncle and cousins, who still run the fair, and most summers he will go back and hang out with them on the road as they work their way round the small towns of the Highlands.

A full-time life in the fairground could never be an option though – Hercher’s love of film-making is too strong for that. It also seems to be a way for Hercher to express his compassion for human beings in trouble or difficulty. At one point, we drive around the new development in Sighthill that has replaced the old, damp tower blocks that were pulled down last summer and every second person we see Hercher knows. He likes them, and they like him.

Hercher has also tried to maintain a relationship with Gary, although it hasn’t been easy. When the teenager was staying in the woods, Hercher would bring him hot food or buy something that he could burn to stay warm. This is how he works, says Hercher – he is not just a watcher, he’s a participant too, and I ask him whether there are similarities between Gary, the chaotic teenager living rough in Sighthill, and Darren Hercher, the documentary film making working for BBC Scotland. Hercher doesn’t particularly like these attempts to look for patterns or narratives, but he says, yes, there are similarities, particularly the lack of structure in their lives. “I’m probably doing what I do now as a direct result of my upbringing – that transient existence.”

There is certainly a personal element at work in Hercher’s new film. When he came to Sighthill to make his first documentary about the area in 2008, he had just moved to Glasgow and was in a relationship. Eight years later, when he came to make the new film, he had split up with his girlfriend and was in a less certain place.

As we drive around Sighthill, past the old graveyard on the hill, through the piles of rubble that were the old tower blocks and around the diggers that are building something new, Hercher talks about how the film was never just about the buildings and the regeneration. It was never just about Gary and Robert and the others. It was about Darren Hercher too.

“It’s always quite strange revisiting something,” he says, looking out over the great mounds of stone. “My life changes as well and that’s the poignancy that I felt. When I came here the first time, I came to Glasgow with a girlfriend and we had just finished at the start of the latest film. So it was like revisiting a certain chapter in my own life.”

He was aware, he says, that places change but so do the people in them. Sighthill was different, but he was different too.

Sighthill is on BBC2 tomorrow [Tuesday, 10 January] at 9pm