AT 11 o’clock on Saturday night on the banks of the River Thames, James McArdle is being mobbed. Fans want selfies; they want things signed. Everybody wants a bit of him. All McArdle wants, though, is a pint. The Glasgow actor has just pulled eight hours in a titanic two-parter at London’s National Theatre in the acclaimed adaptation of “gay fantasia” Angels In America, with Andrew Garfield and Russell Tovey.

Earlier, he’d told me to “come back” to the stage door – it’s what performers say when they invite you to join them for a drink after – but the autograph-hunters have beat me to it.

McArdle shoots me a knowing glance over their heads as he signs and poses. This boy from Darnley knows a journalist from Glasgow is watching him indulge his international public in their excitement over his star status. It’s enough to give the everyday Weegie a pure beamer.

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But not James McArdle who, at 28, has been ready for this since he was five. He settles for a hasty pint on the South Bank, and two sips in, the celebrated actor is just another bloke in the pub talking about work, the telly and what’s happening back up the road.

Neither of us is in any state for an interview at 11pm after a full day of live theatre, so plans are made to reconvene in Scotland.

A week later, having swapped London’s South Bank for Glasgow’s south side, the actor recalls the moment his path was set on a journey that would take him from Darnley to RADA (The Royal Academy of Dramatic Art), and a life of Scottish Kings, Russian classics, Star Wars and epic American stage shows.

“I was always acting things out for my parents and family,” he says over a pint in a Strathbungo bar where nobody gives him a second glance.

“Star Wars, The Lion King, Joseph And The Amazing Technicoloured Dreamcoat ... I’d perform at any opportunity. Dad’s in from work, so I’m going to act out a scene from the Lion King.”

There was serious stuff, too. “I was nine the first time I saw A Streetcar Named Desire,” he says of the Tennessee Williams classic. “I would act out the part of Stanley. I’d be like, ‘Stelllllla … !’ And my mum and dad didn’t know what was going on. A nine-year-old method actor in the living room. Apparently my cousin threw a dish towel over my head once, like you would a budgie, to try to get me to shut up.”

It was the influence of his maternal grandmother, Betty Feeney, that embedded a vocation in her grandson which would see him beat the odds to land a place at the UK’s most prestigious performing arts hothouse.

“She always engaged with me in the story,” he says, warmly explaining that his 84-year-old granny is known around these parts as Bettyfeeney, even to her mates. “It came from her. She looked after me a lot when I was younger and she always encouraged me to make up stories.

“We’d play Thomas The Tank Engine and I’d say the couch was Sodor Island Station and she’d stick to the geography of that with fierce commitment. That’s acting.

“As I grew older my mum would leave beach towels out for us both when she went to work, and my gran would become the willing Robin to my Batman. She completely engaged with me in a way other adults didn’t.”

Barely a decade later, McArdle was standing without a beach towel or a grandmother in front of a panel at RADA fluffing his lines and falling victim to the odds stacked against boys from working-class Glasgow families.

Having pretended to his parents that he was off to stay at a friend's, he caught the bus to London for an audition.

“I was terrible,” he says. “So bad. But I left, went halfway down Gower Street then turned and went back up.”

Charming his way past the secretary, he found his way back into the room where he’d fatally holed his future 10 minutes before.

“They were having soup for their lunch. I said, ‘Is there any chance I can just do all of that again?’ I remember this guy calling me a cheeky little b*****d.

“I remember saying to them: ‘You just eat your soup … and I’ll just start’.”

He hasn’t stopped since. In 2015, McArdle won the Ian Charleston Award for his performance in Chekhov’s Platonov, and has since garnered glowing reviews for his turn in the National Theatre of Scotland’s James Plays as well as Chariots Of Fire in the West End.

Yet he harbours no notion that he should be grateful for breaking into the classrooms of the elite. “I check my privilege every single day. I know I’m lucky for the work my gran and my parents [Jim, a plater, and Carol, a bank worker] did.

“Privilege gives you confidence to believe that you deserve it, to achieve what you want to achieve,” he says. “But then I went to RADA and realised what real privilege was. That’s when I realised I wasn’t privileged at all. I had always assumed equality, it was how I was brought up, being told I was as good as anyone else and that my opinion mattered. I naively thought that class was something that only existed in the film Titanic, and then I realised it was alive and well.

“I had to fight 10 times harder to prove my intelligence with people who are deemed intellectual or high up in the industry. I don’t any more, but I had to learn that there wasn’t equality.

“I never felt inferior. I assumed equality and soon realised that equality doesn’t exist. But that doesn’t faze me. I’m not going to be fazed by a bunch of Etonians. I won’t let a bunch of posh boys stop me.”

He’s not alone in that. The past 18 months have seen names including Greenock’s Martin Compston and Nottingham’s Vicky McClure drive the debate about the issue of class and privilege in the performing acts, which has given rise to a rhetorical battle about wealth, entitlement and the predominance of privately-educated actors within the industry.

McArdle attended St Ninian’s High School, Giffnock, and his early promise saw him find his way onto the register of the prolific talent stable at Paisley Youth Theatre (PACE), alongside names like River City’s Kiera Lucchesi and Sunset Song star Kevin Guthrie.

By aged 13, he was equipped with knowledge about the teachings of dramatic theorists such as Stanislavsky and fluent in the work of Tennessee Williams.

“By the time I went to RADA I knew about things like [the Spanish poet and playwright] Lorca and because of that, the Spanish Civil War. I was being exposed to things that I wouldn’t have been otherwise, and I’m still so grateful for that,” he says.

“There were plenty of those Eton boys at RADA but some of them just couldn’t act. I remember thinking, ‘Why can’t they act?’ There are probably as many in my group at PACE working [as actors] as there are from my class at RADA.”

McArdle is a ferocious and invigorating conversationalist. In the time it takes to neck a third of a pint of Tennent’s, he’s talked about the dumbing down of the working class (“In my gran’s day, working class didn’t mean uncultured. Now we have ITV2 and YouTube”); what lies behind the macho facade of many Scottish men (“Growing up I was confused by what I now understand was fear and a lack of confidence in men. When you’re young, that just reads as aggression”); and his Glesga mates messing around on the set of Platonov after he’d invited them backstage, (“Eight Glasgow boys on the stage afterwards, playing with the props and taking photos. I was sure they’d get me sacked.”) and how he took photos of himself with all his Star Wars toys to persuade his agent to help land him a tiny part in The Force Awakens as rebel pilot Niv Lek.

There’s no need for such arm-bending any more. McArdle’s die seems cast and his career trajectory looks to be anchoring him in London with America surely already calling.

Later this month, he’ll play a lead in BBC1’s Man In An Orange Shirt, a Second World War drama in which McArdle plays one half of a gay couple – one a war artist, the other a British Army Captain – who fall in love in Italy.

The drama, penned by novelist Patrick Gale, also features Vanessa Redgrave and Frances De La Tour, and is one of the main pieces in the BBC’s Gay Britannia season marking the 50th anniversary of The Sexual Offences Act 1967, which partially decriminalised homosexual acts between adult men.

“I think that’s a good thing, but it confuses me a little,” says McArdle. “I didn’t realise it was going to be part of a season called Gay Britannia, I thought it was going to be put on as a mainstream programme, which I was very supportive of.

“It irks me a bit, because it’s a classic romance, and the protagonists just happen to be two men. It’s Gay Britannia, but it’s also just Britannia.

“The thing that saddens me a little bit is that if you have Gay Britannia, then we’re admitting that every other night of the week is Straight Britannia.

“People who live a heteronormative life might feel they are free, but until we live a life which includes equality of sexuality, gender, equality of class, equality of race then no-one is free.

“There’s no freedom at all unless there is freedom for all. I understand there have to be labels when there is still a fight to be had, but that shift has to be cultural and it’s never going to work if you keep dividing people.”

The political resonances of Angels In America, a Tony Kushner play set in the 1980s and first staged in 1991 in response to the advent of the HIV/Aids crisis, are, he feels, as relevant today as they were during the era of Reaganomics and the Cold War.

“I think it’s one of the greatest plays ever written,” McArdle says. “It sums up an epoch in a way that not many other plays do and I genuinely believe that if there’s still a civilisation in 400 years time, then much in the same way we look at Shakespeare now, if people want to know what it was like in the 20th and 21st century, Angels In America sums up what it means to be alive in this age of capitalism.

“It’s always billed as a ‘gay play’. Why? Because it has gay protagonists? Above and beyond that for me, it’s a play about the dangers of capitalism.The characters are victims of a culture which serves to betray them. And that’s what I think about where we are now.”

Audiences around the country can see for themselves when the play is screened at UK cinemas as part of National Theatre Live.

McArdle’s parents have yet to see it, but will shortly make the journey south for the full-day performance in which their son is cast in the middle of a love triangle between a man dying of Aids (Garfield) and a possibly bisexual Mormon (Tovey).

It’s a long way from Thomas The Tank Engine and beach-towel Batman.

“They might shut their eyes at some parts,” he says. “I’m in a Michael Winterbottom film called On The Road and there’s a lot of sex in that. They saw a private screening of it and I heard my mum just went to the toilet. I don’t know if it’s because I’m an extrovert, but I don’t bat an eyelid doing those scenes now. But I feel for my mum and dad. Sometimes I think I must bewilder them.”

On The Road, released this autumn, is British cinema auteur Winterbottom’s erotic music documentary, following the fortunes of British band Wolf Alice while on tour.

McArdle plays a drum technician, a part which offered the chance to perform on one of Glasgow’s holiest of stages.

“I got to set up and play a bit in front of a crowd at the Barrowland. The crowd went mad when they heard my accent. But the experience made me realise that when bands say Glasgow is the best audience, they mean it. Let me tell you, they were the best crowd by far. It was like the Moulin Rouge.”

No doubt Bettyfeeney would have something to say about all that, too. “I feel like I owe her it all,” says James McArdle, the living-room thespian turned darling of the National Theatre. “My manners, my belief, tenacity, compassion – it all came from her. She’s what I measure any moral decision on. She’s very present in me. And it’s going well. Touch wood.”

Angels in America Part 1 and 2 is in cinemas across the UK on 20 and 27 July as part of National Theatre Live. To find your nearest participating cinema visit http://ntlive.nationaltheatre.org.uk

Man In An Orange Shirt will air on BBC 2 on at the end of July as part of the BBC’s Gay Britannia season.