Peter Mullan is known for playing pushers and punishers; men who drink and destroy. There are buckets of booze and abuse in his back catalogue, the cocktail strongest in Tyrannosaur, Paddy Considine’s bleak redemption tale, which opens with Mullan’s character, a raging drunk, kicking his dog to death.

In Sunset Song, Terence Davies’s lush but brutal Lewis Grassic Gibbon adaptation, Mullan plays the patriarch of a Scottish farming family, the Guthrie clan. Gruff, resentful and controlling, he’s at his lowest after a stroke leaves him bedridden and in the care of his teenage daughter (Agyness Deyn). He rings a bell. When she comes to tend him he makes an incestuous lunge for her, collapsing, undone, to the floor.

In comparison, Mullan’s new film, Hector – the directorial debut of writer Jake Gavin – is a relief. He’s the title character, a homeless man hitchhiking from Glasgow to London to spend Christmas in a favourite shelter. He may go through hell to find a roof, but Hector remains an optimist. He deals with tragedy in his past by punishing himself, not those around him. Mullan is fantastic in the film. He creates a world out of Hector’s pain, which he knows something about because he has spent nights sleeping rough himself. Once, when he was 14, he ran away from home for a few nights. Another time, when he was in his 20s, he and his mate, Eddie, camped out in Paris’s Place de la Concorde.

“We had a camping knife and we had to brandish it a few times,” he says. “Some of the more seasoned gentlemen of the road were for robbing us blind. We had big backpacks, camping gear … you can imagine. They were like, ‘Fresh meat!’ They got a shock with two Glaswegian kids who could look after themselves.”

Life on the road is riddled with paranoia and uncertainty, he says. You’re never sure when someone will stumble on a weakness to exploit.

Mullan in Hector. Photograph: PR

“What I remember more than anything is that minute that feels like an hour and the hour that feels like a day,” says Mullan. “You just want it to be over. It’s three o’clock in the morning, you’re freezing cold. You’re so vulnerable. Somebody or something unexpected coming into your bedroom is enough to give you a heart attack, how the fuck’s it going to feel when you don’t have a bedroom?”

Mullan’s rough upbringing has been mapped multiple times by those looking for the real-life inspiration behind his bleaker roles. He was born the second youngest of eight kids in a family ruled over by an alcoholic dad who was violent to their mum. It got so bad that, in what he’s called “a moment of melodrama”, Mullen put rat poison in a cup of tea, took it to his dad and asked him to drink it. “He just laughed,” Mullen said at a Guardian event in 2003. “Because he knew that I never made him tea. If I’d been in his shoes, the idea of my child trying to kill me would hurt, but in his case, he found that amusing.”

Hector - video review Guardian

His dad died of lung cancer the day Mullan left for university. Mullan was ironing a shirt, his dad was lying on the sofa. He heard his dad’s breathing stop. He went over, kissed him, called for his mum and left. In 2001, he described the liberation he felt: “I went out for a dance that night. It was like, ‘Life begins now.’”

He spent part of his adolescence hanging around in Glasgow with a gang with a fondness for knives. A smart kid with a burgeoning interest in Marxism, Mullan didn’t fit in. He left after a while, but wrote about some of his experiences in Neds, his third film as writer and director. Before that The Magdalene Sisters – Mullan’s Golden Lion-winning drama, based on the real-life story of “fallen” Irish girls who were forced into asylums by the Catholic church – and his role as a recovering alcoholic in Ken Loach’s My Name is Joe (which won him the best actor prize in Cannes) had already established Mullan as a multi-faceted talent.

In person, he’s warm and voluble. There’s a big heart in that barrel-chest. He strolls into the room and announces: “Today’s crazy tune in my head is Mandy [by] 10cc,” (the song, released by the pop-rockers in 1976, is actually called I’m Mandy, Fly Me).

“How does that one go?”

There’s a big intake of breath and then …

““Iiiiiiii saw her walk across the water ... As the sharks were coming for me.”

Earworms are a regular affliction. A recent shoot saw him wandering the set, quietly singing the Mastermind theme tune. “How the fuck are you supposed to spend the day?” he asks. “All day in my head like ‘Duhhhh. Duhhhh duhhhh. DUH-DUUHH! I told a few people and they were raging … ‘I’ve had that tune in my head all fucking day because of you!’”

You sense that Mullan’s happy enough in a world of his own. When his emotions are full steam ahead he grips the edges of the table as if it’s a ship’s wheel and sways side-to-side, steering the conversation. Hector’s director, Jake Gavin, remembers waiting for Mullan to stop telling anecdotes so that they could start shooting.

“I was quite nervous to begin with, so I’d wait for him to finish his story before calling, ‘Action,’” says Gavin. “We were losing so much time that eventually I started saying ‘Action!’ halfway through an anecdote. He’d stop. Go straight into character, do the scene ... ‘Cut!’ … ‘So Peter O’Toole and I are in the pub …’”

Mullan in My Name is Joe, directed by Ken Loach. Photograph: Allstar Picture Library

Mullan is switched-on, though. A socialist since his early teens, he was a vocal supporter of Scottish independence and attacked the BBC for what he called “horrendous bias” in its coverage against the Yes campaign. He voted for the SNP in the last election and still has plenty of spite for Labour and Jeremy Corbyn.

“He’s a unionist, so I don’t have a lot of time – no offence,” he says. “He’s no different from the Labour party that told us [before the independence vote]: ‘If go your own way we’re going to destroy you economically.’”

“Every Scot’s got a story to tell about how lethargic and corrupt the Labour party got in Scotland, even though we good as created it. They were always the ones saying ‘You’re going to vote for us anyway, so why should we give a flying fuck?’ They got bluer and bluer under Blair and then Brown. Then you were voting for red Tories.

“Sixty-five of them just voted with the Tories to go along in this idiotic and immoral act of dropping bombs in Raqqa. To achieve what?”

He implies some of the pro-bombing Labour MPs were voting for air strikes to attack their leader. “They hate Corbyn more than they hate Isis,” he says. “Bloody hell! How far are you guys willing to go?!”

He does Hollywood now and then (“the work here isn’t paying”), haunting both parts of Harry Potter and the Deathly Hallows as the Death Eater Yaxley. He joined the small army of Scottish actors hired for Braveheart and popped up as a totalitarian crank in Alfonso Cuarón’s Children of Men, goading the dispossessed into parading their “’fugee face”. On his suggestion Cuarón swapped his character’s weapon of choice to something less obvious, but more threatening. “They have production meetings about what guns get used [in studio films],” he says. “You know you’re working with the real genuine auteur stock when you say ‘Can I change the gun to a stick?’”

Soon he’ll appear in Andy Serkis’s Jungle Book: Origins, albeit behind a shroud of CGI as Akela, the chief of the wolfpack that take in Mowgli. But he doesn’t get recognised much, even when front and centre. The opening scene of Hector was shot with the crew at a distance and Mullan in character walking across a car park.

Mullan with the SNP’s Nicola Sturgeon during the independence campaign. Photograph: Jeff J Mitchell/Getty Images

“I do remember saying to Jake and the guys: ‘Listen. I might get recognised here. If it happens I’ll keep playing until the next person comes along who doesn’t recognise me.’” As it turned out, he wasn’t recognised once.

Fame’s much more of a burden for the young and pretty anyway, he says. “If young, handsome Ewan McGregor walks into a pub and someone’s girlfriend says: ‘That’s Ewan McGregor!’ the guy’s going to feel undermined. I’ve been in a pub with Ewan and seen that happen.”

“I’m never going to do that to anyone. Guys like Ewan or Colin Farrell, that’s something they’re acutely aware of and they handle it and get to fuck quick. I can think of nothing worse: just by virtue of being you, you’ve made somebody feel less happy about themselves.” He grips the table, leans back and gives a broad grin. “I’m not a threat to anybody.”

• Sunset Song is currently on release in the UK. Hector is out on 11 December.