A few years ago, 29-year-old Alex Farrell was another struggling young actor in Hollywood who had drifted to Los Angeles from London. “I realised it wasn’t what I wanted and LA didn’t afford me the opportunity,” he says. “It’s easy to get lost in the Hollywood bubble. I decided I was more interested in being behind the camera.”

On a cold, rainy morning, sitting in his two-room cabin behind his mother’s house in Kent, he is drinking pear juice and smoking roll-ups, telling me that, “after seeing the horror and despair refugees face as they seek safety and shelter”, he felt driven to tell their stories.

The result is Refugee, a film shot over an eight-month journey showing the impact of war through the eyes of the people who live through it. “It was awful, heartbreaking, terrifying, and I was naive to think I was ready for it,” he says. “It was bitterly cold. The rain was torrential. People were suffering from hypothermia. Children were frozen, screaming and crying.”

Yet Farrell was struck by their resilience and optimism. “Many had been on the road for months, yet they still smiled, shared what little food and water they had, which was inspiring given the losses and atrocities they endured. Amid the chaos, I was also amazed by the unconditional kindnesses of volunteers and soldiers working around the clock to help them.”

All this is reflected powerfully in the film; poignant shots of pelicans flying over midnight-blue seas contrast with wave after wave of displaced people suffering from hunger, exposure and sickness. After losing everything, forced to leave their homes and country, with nowhere to go and surviving perilous sea crossings, they are subjected to beatings and tear-gassed by police at gridlocked borders. In the midst of it all, Farrell keeps filming and often at great personal risk. As he walks with them through 10 countries, he is incarcerated by the Macedonian military and beaten by the Croatian riot police.

The idea for Farrell’s film came to him several years before when he returned from LA to London, feeling lost but determined to do something meaningful with his life. “I went to Kenya and spent three months working in a wildlife reserve documenting poaching, and to Peru to shoot a children’s charity working up in the Andes; and from there to Norway, where I lived on a boat in the north Atlantic ocean, photographing onboard a sustainable fishing boat. That’s when I saw the newspaper about the Syrian migration. I was excited, sad, intrigued.”

At the end of that summer, he was chatting with friends in a London pub when the conversation turned to the crisis and the grim encampment that was the Calais refugee camp.

“We talked about how this place was only 22 miles away from the British coast and a different world,” he says. “I couldn’t believe what was going on; it seemed medieval. So we went to help with some money that we saved, bought building materials and were putting up temporary wooden shacks for new arrivals.”

While volunteering there, he met people with unimaginable stories: their houses blown to bits, they’d lost entire families, seen friends shot in front of them, and dug children, gasping for life, from the rubble. “One guy, David, fleeing from Aleppo, had managed to break out of an Isis stronghold with his wife,” says Farrell. He’d been tortured and had horrific scars. He crossed the ocean in search of Germany and ended up trapped in Calais. He’d said to me: ‘You need to go to the beginning, Alex, if you’re serious and as passionate as you sound. Get as close as you can, without dying, and document it on a human level.’”

Returning to the UK, Farrell assembled a small crew of filmmaker friends and a producer, Kurt Engfehr, who had worked on Bowling for Columbine and Fahrenheit 911; he took his mother’s beaten-up old minivan and drove it all the way to Turkey. “The first thing I shot were the smugglers operating under the cover of darkness. I had no idea what I was getting into.”

At the time, which was the peak of the crisis, human trafficking was flourishing, he explains. Many of the smugglers, often part of dangerous criminal gangs, exploited the Syrians’ plight making thousands of pounds off each overcrowded rubber boat and sending them on potentially deadly trips.

One night, around midnight, driving up a hilly cliff with the Aegean Sea on one side, they saw hundreds of taxis coming towards them. “It was the refugees,” Farrell says. “They’d fled through the Syrian deserts across to southern Turkey.”

Hiding in the shadows, he started filming them being dropped off at the shoreline. “I was with a sound guy, a translator, a photographer and a guy who was good with tech. We were sort of building a spy operation, parked in this deserted seaside town. All the lights suddenly came on and these seafront shops opened up. People started getting rubber rings and life jackets. Then ominous- looking men were loading them into the boats. That’s where the film starts, as they’re crossing the ocean.”

The documentary focuses on the Alali family, separated by borders, and interweaves the broader story of millions of Syrians whose lives have been upended by desperation and violence. After travelling 2,000 miles in search of asylum for her family, Raf’aa Alali was stranded in Germany by visa restrictions and red tape, while her husband, Nazem, and their two sons, Hamodi, 10, and, Ahmed, eight, were stuck after the borders closed in a refugee camp in Greece. They’d run out of money and medicine, and the youngest child was gravely ill.

“They were living on bread,” says Farrell. “Hamodi had stomach problems, a viral infection, his eye was inflamed from filth and sewage. Frakapor is next to a sewage plant and notoriously the worst camp in Greece.”

On his first day there, Farrell encountered hostility from the camp guards. “I was smashed against the fence and warned: ‘You dare bring your camera here again, we’ll smash it and smash you up, too.’” That meant smuggling in two video cameras and uncalculated risks. “I gave them to Nazem and the boys and told them: ‘You have to film your life here, but don’t get into trouble for it.’ We went through all the consequences. They had nothing to lose at this point.”

That intimacy and the family’s complete trust in Farrell, and willingness to film their own raw and emotional struggles is what makes the movie gripping. When Nazem asks his younger son what he remembers of Syria, he says: “Every day we had bombs in our town and we couldn’t play outside.” When he asks his brother what he would like to see in Germany, he replies: “Mama… only Mama.” At one point, after Farrell gives them his phone to call her, the little one says: “Mum, I miss you.” And hearing her sobbing, he tells her: “We’re sitting by the seaside”, while the camera pans to the woeful sight of the bleak camp.

Like many of the children, they bore the mental scars of the war they’d fled. “The next time I saw them, they’d been transferred to a military camp in Thessaloniki, and the boys seemed older and quieter,” says Farrell, who shot the footage on successive trips to Greece. “They had turned into little men very quickly.”

On the last day of filming, as he prepared to leave the country, Nazem broke down on camera. “He sat by the side of a building, defeated, and he said to me: ‘Alex, my brother, please take my boys. They’re going to die here in this camp. You need to adopt them.’

“I said: ‘If I could, I’d take them and get you over as well. It’s not that simple.’ But he continued begging me, crying his eyes out.”

Farrell shows me pictures of the boys on his phone. “I miss them,” he says wistfully. “I don’t know if you’re supposed to get that close to the people you’re documenting, but I fell in love.”

Soon after finishing the film, Farrell says, he found out that the family had been reunited and happily settled in Düsseldorf after almost three years apart. He acknowledges a quiet pride in keeping the divided family in touch with each other by delivering video messages between them during the separation. Raf’aa and Nazem recently asked Farrell to be their children’s godfather and they regularly Skype and send him photos of their new life in Germany.

“The hardest thing for me to witness on that journey,” he says, “was one day on the final leg in Austria, watching a man who reminded me of my father. He was a respectable businessman, wearing a suit, so he must have been carrying it the whole way, and he was shaving his beard in the reflection of a puddle. I just started crying. I didn’t realise why, but the enormity of it all hit me so deeply. It was really sad. This guy needed to feel like a person, with morals, dignity, class and rights. I thought: ‘Would my family survive that journey?’” he says. “Look at where we live in the Kent countryside. If we were being bombed here, would we make it all the way to Syria for safety?”

His mother, Deborah Tarry, a landscape photographer, told me: “The experience has changed Alex.”

“That’s for sure,” he says later when I mention this to him. “It’s difficult to say how because no one feels themselves changing. My priorities have absolutely shifted,” he explains. “When I came back I was closed off. Only because I was devastated by the world I was living in.”

With the conflict raging on, killing almost 500,000 civilians in the seven years since the fighting began, Farrell sees “a lack of humanity and understanding” at the heart of the issue as attitudes toward refugees harden.

“Most of the people I spoke to about this crisis, especially in the UK, said: ‘Wasn’t Syria always an apocalyptic wasteland anyway?’ And I had to say to them: ‘No, it was one of the richest, culturally diverse nations on the planet,’” he says of the once-peaceful country that was known as “the hidden jewel of the Middle East”.

“We could have done so much more. For us to accept refugees is a conscious choice,” he adds. “The other countries had migrants passing through them and were overwhelmed. Just because we have a massive sea between us it doesn’t exempt us from responsibility.”

Farrell hopes his film will go some way towards shattering some of those misconceptions. “It’s important that people understand Syrians want the same things that we all want: to live peacefully, with a sense of purpose and stability for their families, and without the constant fear of bombs and being terrorised. These people need our compassion.”

Refugee is released in the UK on 1 June. Watch the trailer at vimeo.com/183235071