The night before our interview, I watch my favourite Gary Lewis scene again.

He won a Gijon International Film Festival best actor award for his role in the film, Orphans, but he could have won it for this scene alone.

Briefly: Lewis plays Thomas, the eldest of four siblings. Their mother is being buried in the morning. Thomas alone keeps watch over his mother's coffin in the church on a dark and stormy night during which his brothers and sister, for a variety of reasons, go off the rails.

Come the funeral, Thomas insists that he carry the coffin on his own. Warned by the undertaker that he will find her too heavy, he utters the immortal response, "She ain't heavy - she's my mother," and obstinately loads the coffin on his back. After a few yards of grunting in pain he collapses face-down on the ground - where he remains, silent, even after the mourners have filed past him and the coffin has been lifted up and borne grave-wards.

Glasgow-born Lewis hadn't been in films too long by the time Peter Mullan directed Orphans, which came out in 1999. Since then, he has worked for Martin Scorsese, and for Ken Loach in Carla's Song, Ae Fond Kiss and My Name Is Joe. He has starred in a heavily garlanded film, Billy Elliot, as a striking miner whose 11-year-old son dreams of becoming a ballet dancer. Added to this, he has done a substantial amount of TV, including a role in the high-budget, yet-to-be-broadcast American TV series, Outlander, which was filmed over here.

All of which is a roundabout way of saying that when one of our finest actors describes his most recent project as "one of the best things I have done," it means something.

The project is Glasgow Girls, the latest manifestation of our continuing fascination with the Drumchapel High School pupils who, back in 2005, waged a successful campaign to prevent a Kosovan Roma classmate, Agnesa Murselaj, from being deported, after she and her family had been detained in a dawn raid.

The hour-long drama, directed by Brian Welsh (whose CV includes Charlie Brooker's Black Mirror), is being screened on BBC Three on Tuesday night. Lewis plays Euan Girvan, the highly popular teacher who mentors the girls in their campaign.

Long before Lewis heard about the film via his agent, he had already seen a much-praised National Theatre of Scotland stage production - also called Glasgow Girls -about the Drumchapel teenagers.

"I was on my feet, cheering, at the end," he says of the stage show. "I was so moved, exhilarated. It was a fantastic production: its energy, its music, its heart. I had no doubt whatsoever that I wanted to do this."

The shoot for the new BBC drama was very short, he says. "They had a lot of ground to cover. Brian is a great director. The script was evolving and being rewritten as it went along. We had a small period of rehearsal, myself and the girls, and we explored some themes and ideas, talked about stuff, suggested changes and came up with things."

Lewis has "enormous" respect for the original Glasgow Girls. "My admiration for the girls who played them is just as high," he adds, "because it was a really terrific piece of ensemble acting. It is one of the best things I have ever been involved in. There was a total commitment to telling this story well. Every one of those actresses I thought was terrific."

He was impressed by the cast's commitment to their roles, coming as the actors did from diverse backgrounds and with varying levels of experience. "They brought a lot to it, they did a lot of research, and we just worked so well together. Sometimes you can be involved in a project where there are stupid sorts of games being played. People believe they deserve a certain status or something.

"But in this, the focus was on telling the story and on occupying the shoes of the girls in that situation. It's been a while since I've experienced such across-the-board commitment and generosity. The girls were so giving. There was nothing about, 'How do I look?' or 'Is this the best light on me?' There was no vanity, just generosity."

The drama features occasional musical interludes, and despite initial scepticism, Lewis gradually recognised their emotional power. "I'm convinced it was the right thing to do, because music was so important to the girls, and the way it's filmed, the music draws you into their predicament."

One skilfully done segment underlines the girls' determination. They have been campaigning industriously but now a letter arrives from the Home Office. They open it and read it. The deportation is to go ahead. Agnesa and her family have no right to remain. The girls are devastated. "I'm so sorry, girls," Girvan (Lewis) says, almost in tears. "We've lost."

Agnesa's close friend, a Somalian named Amal (Letitia Wright), too upset to speak, leaves the room. We cut immediately to the next scene: Amal, shortly to be joined by another of the girls, Roza (Aruhan Galieva), begins to sing a heartfelt version of a Pink Floyd song. The song is Wish You Were Here. At that moment, you realise there is plenty of fight still left in the girls. They're not done with the Home Office and the UK Border Agency just yet.

Nine years have passed since Ewelina Siwar, Emma Clifford, Amal Azzudin, Roza Salih, Jennifer McCarron and Toni-Lee Henderson campaigned to keep Agnesa in this country. Their achievements were later recognised when they won the prize for Public Campaign at the Herald Diageo Politician of the Year Awards.

Today, all seven girls are doing very well. Agnesa was granted indefinite leave to stay in Scotland in 2012, and is currently caring for the elderly. Lewis recognises the importance of what they achieved.

"When I met them, and others who were involved in that situation, they would often repeat a phrase: they just couldn't accept it.

"They were determined to change it. They were going to get their friend back, they were going to stop these dawn raids. They were going to stop children getting dragged out of their beds.

"The stage play made you aware of some of the more personal aspects of the girls' lives. I started wondering - was there something special about the chemistry, the heartfulness of the local girls, and the spark that was brought by the girls from other parts of the world?

"What is clear is that they just weren't having it. They hadn't absorbed the hateful propaganda, the racist crap [you might read in the press]. They hadn't absorbed the dominant ideology about people who aren't white, that their lives don't matter as much as ours, that they're really not important if they're not white.

"The girls were solid in their friendship. Their attitude, and that of the community, flies in the face of the uninformed position, which you can hear every day.

"It's very easy to be in the gang which attacks asylum seekers and vulnerable people. You get invited to join every day, and if you forget your script you get daily reminders. But it takes more effort, more heart, to stand up to it. There's a lot to be learned from these young women."

Giving Lewis the role of Girvan was an astute piece of casting. The girls' efforts chime with causes he has campaigned for (or against) for much of his adult life. I mention something that Girvan has said: that he and Lewis share a passion for social justice.

"Absolutely," he says. But he has always remained low-key through choice. He is genuinely surprised to learn that his name featured in a story a decade ago about efforts to free a trainee Nigerian priest, John Oguchuckwu, who had previously lost his appeal for asylum in the UK. Thanks to the intervention of Lewis and the others, Ogychuckwu was free to live with a family in Ardrossan.

In 2005, as the Drumchapel girls were finding their voice, Lewis took part in a documentary that called for the closure of the Dungavel detention facility. A glance through the cuttings also reveals that in 2001 he and other actors took part in a Unite Against Racism And Poverty march through Glasgow.

During the miners' strike of 1984-85, he joined the picket lines in support of the miners, and was part of the East End Support Committee, a Glasgow-based organisation which sought to help the strikers. "We'd raise money, and miners would come up from Ayrshire and stay at my house," he said in a later interview, "but still, you don't know what it's like to be in that state."

"Small acts of solidarity to help people" is how he describes such activities now. "I think it's important you do that, even with small acts. A lot of people did so much, you know? Because of the nature of my work, because I had to go here and there, what I did was relatively small."

Maybe so, but there were times when Lewis showed his compassion and went a lot further than most of us would. "I took people in," he says of something that happened eight years ago. "I took a couple of Somalian asylum seekers into the house and put them up, because they had nothing. I'd seen the other side of the ticket, where people were claiming that the asylum seekers were getting everything. I was close to people who had nothing, you know? Absolutely nothing.

"When you witness this and you try to get it across to people, you see how pernicious the campaign of hate, anti-asylum seeker, anti-immigration, really is. Basically, it's a racist campaign which permeates the media and the very policies of the state."

Lewis, who is from Easterhouse and is an enthusiastic supporter of Scottish independence, came relatively late to acting. Born Gary Stevenson in 1958, he was in his thirties before he joined Raindog, the theatre company established by Robert Carlyle, among others. With Raindog, and other companies, he performed in such notable plays as One Flew Over The Cuckoo's Nest, The Grapes Of Wrath and The Birthday Party.

He graduated to short films, then television, then full-length films. He has appeared in a sizeable number of movies shot or set in Scotland: as well as the Loach trilogy and Orphans, he was involved in Mullan's Neds, and John McKay's Not Another Happy Ending.

Further afield, he lent his presence to Scorsese's riveting Gangs Of New York, as a lieutenant to Bill "the Butcher" Cutting (Daniel Day-Lewis). He had previously been nominated for a Bafta best supporting actor award for his role in Billy Elliot, in which he was superb as a father who is initially aghast at his son's dream of making a career in ballet, but who, once he becomes aware of the boy's talent, offers him his unequivocal support. More than a decade later, Lewis and his "son" in Elliot, Jamie Bell, were reunited as Edinburgh detectives alongside James McAvoy's nihilistic Det Sgt Bruce Robertson in Jon S Baird's film version of Filth.

Lewis is engaging company. And, it seems, he wouldn't mind sharing a pint with the teacher he plays in Glasgow Girls. He didn't have much chance to talk to Euan Girvan before shooting began. "I read an interview with him, which said he loved poetry, Burns, Alasdair Gray, Lewis Grassic Gibbon …" He laughs. "My name - I chose the stage name of Gary Lewis because of Sunset Song [adopting Grassic Gibbon's first name as his stage surname]. And when I told him that …" He laughs again.

As Lewis departs, he is still enthusing about Glasgow Girls. I mention that, more than once, we see a plane traversing the screen in the skies over Drumchapel. It seems like a shorthand symbol for the ominous fate that awaited many pupils at Drumchapel and other parts of the city - pupils whose families had hoped to win asylum here. He nods. "Playing the part of the teacher, you were always aware when you saw an empty chair in the classroom, and heard an aeroplane, that things were connected," he says. "There was a reality that people could disappear." n

Glasgow Girls, BBC Three, July 15, 10pm.