IS James Cosmo’s mind these days wandering a little in a far off galaxy? For years, we’ve come to believe this 6ft 3in sandy-haired warrior is indelibly marked by integrity, authenticity and, thanks to his Braveheart appearance, blue face paint.

But in recent times we’ve had to deal with this Clydebank crane of a man wearing a feather jacket in a bank ad. And don’t we all hate banks? Just to add to the national bewilderment, Cosmo joined the cast of fame seekers in the misnomer that is Celebrity Big Brother.

It’s like discovering William Wallace had forsaken vengeance and taken up tiddlywinks.

“Well, I have to fess up,” says the actor who grew up in Clydebank and once worked in a shipwrecking yard. “The money on offer from Celebrity Big Brother was very good. But it was also fascinating to see reality television first hand. And I wanted to test myself and learn a bit about myself in the process.”

Yes, James, but you have to check you dignity at the door. And be manipulated. “That is so true,” he agrees. “They try to create situations that will wrong-foot people, make them feel uncomfortable and vulnerable. They want to play people against each other. But I think a lot of the people in there were pretty desperate for fame so it was easily done.”

Cosmo came up against telly cleaner Kim Woodburn, who behaved like she were a tin of Pledge short of a full cleaning kit. And Jedward, two creepy little specimens of human life.

“I found it all quite depressing in a way. But I’m glad I did it. I’m fascinated by the human condition and how we behave under pressure. But I wouldn’t do it again, being in isolation with some very odd people.”

He adds, grinning: “I realised it was pretty unusual for an actor in my position to go in there.” What did he learn about himself? “That I was a bit more tolerant than I thought.”

Cosmo has no regrets about the Bank of Scotland ads, a homage to his Game of Thrones character Jeor Mormont, who spouts nonsense dressed up as pearls of wisdom. “The money was great, I enjoyed making them and they’ve been quite successful.”

James Cosmo is said to be worth £6m, if the glossy TV mags are to be believed. But he admits the choice to keep working was prompted by the desire to make sure his two sons, aged eight and 23 years old, are well provided for. Not that Cosmo is ready to shuffle off. He’s 70 this year but still looks more powerful than the horses he rides in Game of Thrones. No, he’s making sure his relationship with his boys is very different from the one he had with his father, actor James Copeland.

“We had a very typical west of Scotland relationship,” he explains of the familiar familial tale. “I’m sure he loved me but of course he never told me. And it was a strange upbringing. I lived in two very different worlds. My mother, sister Laura and I lived in Glasgow and then when I was eight we all travelled to London in a gypsy wagon to live with my father, who had landed West End work. [The journey took two weeks.] Then I entered this world of the theatre, which was all new.”

It was bewildering and fascinating. Young James played cricket on Hampstead Heath with Sean Connery while his father (who also wrote songs, including These Are My Mountains) drank in the pub with Peter O’Toole. “Then when I was 11 we moved back to Glasgow.”

His parents didn’t split up, as such. “My parents were sort of together, my mother worked at Singers sewing machine factory in Clydebank and my father in the West End.” He adds, with massive understatement: “It was all very strange for me.”

Having been introduced into a world of cravats and afternoon gin and tonics it must have done his sandy head in to be returned to Fifties Glasgow, like an empty Mackesons beer bottle from a run-down boozer. Was he an angry young man as a result? “A little bit,” he says, with an accordant grin then adds: “I’ve always been my own man, for good or bad.”

Cosmo admits he was far from a model schoolboy, and in fact he and his school parted "by mutual consent". But if his head wasn’t in school it certainly didn’t sit comfortably under the bunnet he wore to work at Arnott Young shipbreakers in Dalmuir.

"It was horrific. Dickensian. The men would burn the grey paint off the battleships and had not a f***ing thing to protect them, just hanging off a pulley inhaling lead fumes. I thought ‘Jeez, I don’t want to be doing this all my life.’”

But what? He had looked through the window of the acting world, where you didn’t die from paint fumes, and dreamt of having a go. “I wasn’t scholastic. I wasn’t going to make it in Business or Law.” But his father wasn’t encouraging. “I think he realised what a tough life acting was. Yet, he introduced me to the director of Dr Finlay’s Casebook (in which his father was appearing) and said, ‘After this, you are on your own, son.’”

It was sink or swim. The teenager swam. And changed his surname to Cosmo, the middle name of his mother, Helen. Did he feel a natural affinity with his new world? “No, and I never have,” he says, astonishingly, given the success he has enjoyed. “I always felt an outsider in the world of the theatre. I never went to drama school and since then I’ve always felt I was getting away with it. But I’ve managed to have a career.”

Over the years, he worked in stints and then went back to the building site or took bar work. What gave him the resolve to keep going when his parts were small, in the likes of Battle of Britain in 1969, or Take The High Road in the Eighties? “I’ve never held acting in any great esteem but at the same time there was nothing else I felt I could be really good at. But I did something when I got to play great parts and I did my best when I played the poor parts.”

James Cosmo’s life changed in 1994 when his second wife Annie, a TV production secretary, picked up the phone at their Surrey home. She called out: "It’s Mel Gibson on the phone." Her husband didn’t believe her and was reluctant to take the call. But he did and the role of Campbell in Braveheart took Cosmo’s career into a new orbit. The couple bought a house with the fee which he was tempted to call Casa Gibson. But on a less material level was the role an endorsement, approval for the life he had chosen?

“No, I just felt incredibly lucky,” he admits. “I never thought, ‘This is my time. I’ve worked hard for this and I deserve it. I just thought, ‘I hope I don’t screw it up.’ And to this day, every time I walk onto a film set, I feel I’m going to get found out when someone says ‘You can’t actually do this, can you?’ and I end up saying ‘You’re absolutely right.’”

Cosmo’s currency was high. Casting directors for the likes of Troy and Soldier, Soldier realised he had weight, although he was all too often cast as the heavy.

“For years I was defined by my size. If I wasn’t a thug I was someone on a horse hitting someone with a sword. I’m not saying I didn’t like riding around pretending to be a tough guy, and I accept you do get typecast. But as I’ve gotten older it’s been great to show I can do a bit more, to play a range of roles.”

That was certainly the case in 2001 film All The Queen’s Men, in which he co-starred alongside Matt LeBlanc and Eddie Izzard. The actors had to dress up as women in this hokum spy drama. Not to be insulting, James, but you don’t lean easily towards looking girly in real life.

“No, I’m not insulted at all,” he says, laughing. “But all I can I say about this film was it was a German comedy. It was the most humourless piece of film ever made. And what I do remember is Matt LeBlanc looked quite hot as a woman. Oh, and the make-up people couldn’t get Eddie Izzard out of the chair. He kept on trying different lipsticks and make-up shades.”

He adds: “But I hated getting dressed up as a woman every day. And what women have to go through to be attractive to us numpties, well, it shouldn’t be allowed.”

As age increased, so too did the more thoughtful roles, in the likes of Trainspotting. (Cosmo had become an autodidact in real life, a man not averse to quoting Greek philosophers.)

Perhaps the wisdom was noted when he auditioned for the Game of Thrones role. But how does he deal with the subsequent fame. “I just smile,” he says, smiling. “I don’t take it too seriously. I find it charming when I get the plaudits. And when other actors say, ‘I keep getting recognised’ I think, ‘Bully for you.’ You see, not many people say to a plumber, ‘You are a really great plumber.’ But he’s doing a job the same as I am. And I get paid really handsomely. So I consider myself really privileged.”

He enjoyed working on his latest role, in the “re-telling” of Whisky Galore. “It’s a nice homage to the romanticised idea of the Scottish Highlands. It was delightful to do, although not in any way connected to reality, even if the story of the SS Politician going down were true. And it’s always great to go back home to work on a movie.”

Was he concerned a remake wouldn’t work? “Yes, there was a notion in my head, why remake such a delightful original, and I wondered where we could get those faces again, of the islanders shot in the film. The original was of its time and period. But this is a homage to that whole period.”

The 1949 Ealing movie was shot in what are often seen as more innocent times. We’re all cynical now and we live in a global village, don't we?

“Yes, and that has been addressed, the story brought up to the 21st century. As a result they are two different films, with the story revisited.”

Cosmo plays a clergyman. “In the film the minister is quite gentle but I’m sure the reality was very different. The role got me thinking about the power of ministers in those communities because I find the Free Church to be not particularly pleasant, the idea of holding a community in sway.”

Did he grow up feeling the wrath of the church? “Oh, yes indeed. I felt the Catholic Church and the Free Church to be something of a malevolent influence. They exercised a degree of control which I found offensive and not very nice.”

He grins. “As you can tell I’m not that keen on organised religion.”

Has his peripatetic, oscillating career, and life experience, the relationship changes, given him a more empathetic view of his father? “Absolutely. They were men of their time and it would have been very hard for them to verbalise their feelings. I know my dad loved me desperately, even if he never told me that.

“Later on in life you understand the challenges all dads face and you have a bit more compassion.” He says: “It’s a bit like Sophocles said of growing older and his libido diminishing. He said it’s like being unchained from a mad man. When you get rid of all that testosterone you start to see the world in a completely different way. You realise what a clown you were when you were a young man. But then all men are.”

The actor may be providing for his boys but that doesn’t mean he is ready to shuffle off to that great HBO heaven. “I don’t feel like 70 should feel. I’m full of life and keen to work. Seventy is the new forty.”

In an excited voice he reveals he’s made a movie which is his “best ever,” The Pyramid Text, about an aged boxer and his estrangement from his son.

“Looking back on my 53-year career it’s the one thing I’m actually proud of. It’s the one thing I can say, ‘That’s my performance, and I’m so proud of it.”

James Cosmo’s mind is still very much in this galaxy. The humility in the man is as visible as the giant sword in Jeor Mormont’s hands. “You always have self-doubts,” he says. “But then you don’t want to go onto a set with a great big ego. When I’m on a film set I feel very nervous and very vulnerable.

“But having said that I’ve found satisfaction in something I never found anywhere else. What I think the industry has given me is freedom. I’m not tied to anything. I’m always my own man.”

Life, he says, is about simple pleasures. “I feel incredibly blessed. What I’ve achieved is really quite astonishing. But as the French philosopher Michel de Montaigne once said, “I wish death to find me planting cabbages in my garden.’ That’s me. A nice little cottage in Spain where I could grow vegetables.”

Whisky Galore (PG) is in cinemas from May 19