"Mmmmmmm." James McAvoy closes his eyes as he groans. "It's fucking awesome, it's amazing," he continues, his words eager yet slow. "I could lose myself to it. I could have an affair with a Shake Shack burger."

Er, quite. It's hardly a conversation you'd expect to have with a Hollywood actor, but then James McAvoy is not your ordinary star. As he launches into a graphic description of his favourite burger in town, he swears like a trooper, banters like a mate down the pub and wears his talent with a refreshing lightness.

Theoretically, McAvoy's travelled from his home in Crouch End to this pink-cushioned hotel suite in Soho to talk about his third stint as the young Professor Charles Xavier in the latest film based on the Marvel comic book series of X-Men. Yet he doesn't seem too prepped for the chat. With a wave of superhero films hitting our screens this year – some decent, some not so much – I want to know what will make X-Men: Apocalypse stand out from the crowd. So does McAvoy.

"Er, we've got a big blue guy trying to destroy the world. And it has an amazing cast of proper actors, which I think is something that you don't always get. What am I talking about?" he says, shaking his head. "The other films have great actors in them as well. So what is it, then?" he asks me.

McAvoy may not churn out the obvious, scripted media-friendly answers, but that honesty is sure to have contributed to his phenomenal success. Glancing at him – jeans, scruffy jumper (with holes), nondescript Casio watch (and I get the sense that this is him smartening up) – he appears an unlikely Hollywood actor. In fact, he seems most at ease when detailing his favourite patty and bun combination. "I like them really greasy," he tells me. And then he groans again.

All this is good fun for the bloke whose big break in acting came in the form of Steve McBride – a young lad in Brit-born comedy Shameless. Twelve years later, and the Scottish actor's X-Men role has contributed to the series' US$3.8bn worldwide box office takings. The next film, he explains, is still about the world ending, the missiles, the bad guys wanting to kill people because they just want to.

"But the thing that moves all that stuff, the thing that makes all that stuff happen, is the fact that you've got this bunch of characters who've known each other for ages, and they've wronged each other; they've righted each other; they have sometimes fucked each other; they have tried to kill each other; they have saved each other." It's that emotional connection and power that he believes pushes along all the "crash, bang, wallop and duh duh duh" (he fanfares an imaginary trumpet in my face) that makes it different. "Is that it? Maybe?" he asks me again.

Before I meet McAvoy I ask some friends what they think about him. The women sigh wistfully, eyes glazing behind what I can only imagine is a medley of erotic fantasies and hand-in-hand strolling through sun-drenched fields. And the guys? Gushing, uncharacteristically nice compliments spill from their mouths: he's "class"; "a good lad"; and a "massive sploosh" (try Googling that last one). He's also appeared on numerous 'world's sexiest man' lists – GQ, Empire, Glamour. "Have I?" his Glaswegian slur rises a few octaves. "You know you have," I tell him.

So does he think shaving his head for the film will finally get him to the number one spot? "No! No, I don't," he says, laughing loudly, animatedly, before he continues. "In fact, I've met quite a lot of people who've been fans for a long time and they're like 'How you doing?' and I'm like, 'Hey it's good to see you again' and they're like, 'When are you going to grow your hair back?'.

Sure, it's not his finest look – his signature pale-blue eyes are a little lost against the brutal buzz cut, his dark circles ever so slightly accentuated by the lack of hair. He looks… normal. And a little intimidating. "I'm not joking: you get far less shit," he's excited again. "Not this length so much," he says, rubbing his head. "It's starting to lose its power – but when it's a couple of days shorter than this you get a lot less comeback if you cut someone up. If people start beeping you in the car and then they see you, they're like, 'Oh fuuuck, he has a skinhead'. It's great." Cue a satisfied grin.

I don't think I could have made it working in the City. I'm pretty shit with numbers, for a start

Raised by his grandparents on an estate in Drumchapel, Glasgow, McAvoy briefly flirted with the idea of joining the Navy, or becoming a priest. Could he have made it in the City, then? "Nah, I don't think so," he tells me without so much as a brief hesitation. He's "shit with numbers", he admits. But also would struggle with the social side of office life: "If I'm in a situation where I feel like I need to get along with people, I just check out. I walk away or slightly disassociate myself by doing the wrong thing on purpose."

Yet you wouldn't think it today. In every word and movement there's an extraordinary charm, a trait that makes men and women long to take him for a cheeky pint. He's an actor of chameleon abilities – one who's nailed a range of roles with a commanding on-screen and on-stage presence. In the film adaptation of Irvine Welsh's Filth he plays a misanthropic policeman who forces a 15-year-old girl to give him a blowjob in the first few minutes of the film. In contrast, he's the wholesome, if slightly selfish, moral compass of The Last King of Scotland, a role that saw him playing Ugandan dictator Idi Amin's doctor. Meanwhile, his portrayal of Macbeth at London's Trafalgar Studios went down as well as his voiceover gig in the family animation Gnomeo and Juliet. He plays the dick as convincingly as the darling.

And then there are the ladies. Who can forget that intimate, up-against-the-bookcase romp with Keira Knightley in Atonement, the lusty kiss with Angelina Jolie in Wanted, and the subtle seduction and prompt unexplained ditching of Anne Hathaway as Jane Austen in Becoming Jane? OK, not everyone remembers that last one.

His most recent female co-star, Jennifer Lawrence (who stars alongside McAvoy as Mystique in X-Men: Apocalypse), made headlines in virtually every women's magazine last year for speaking out against the gender pay gap in the film industry, after the Sony Pictures hack revealed the actress was paid a mere smidge of her male co-stars' takings for her performance in American Hustle.

McAvoy quickly turns serious; it's an issue that gets him pretty fired up, too. Perhaps his reaction is accentuated by being married to an actress – his wife Anne-Marie Duff, who he met on the set of Shameless, also boasts a solid career, and recently starred in Suffragette, a weighty film about women's rights. Or maybe he's just a fair guy. I suspect it's both.
"Jennifer's been instrumental, and should be applauded for being so outspoken. She has a unique ability to speak her mind and people respect it," he tells me. "When a lot of people say what they think, people see that level of openness as an opportunity to attack. Whereas with Jennifer it's a reason to admire her. I am so full of respect for her using that ability."

It's an issue, however, that he believes needs to be taken on a job-by-job basis (a smaller part, less time on set), yet one that's ultimately spiralled from the "deep-seated and deep-rooted chauvinism" that's existed since the first film was made in the 1890s. Today, agents will fight corners to secure the best pay for their Hollywood clients, in a ruthless fashion – it's a process McAvoy describes as "cut throat."

"It's not called 'show friends', or 'show feminism,'" he tells me, leaning forward in his seat, "it's called 'show business'. And that's what you're fighting against the whole time. Because it's a fucking business."

It's not called 'show friends' it's called 'show business', and that's what you're fighting against

"And where we get confused is because it's a business based on art. So art should be equal, and art should be for everyone. But we've turned it into a fucking business. Which isn't a bad thing," he opens up his hands and sits back as he speaks, "because it means we make loads of fucking great films."

So what does the future hold? "I think we're talking about it, and we're fighting for it. And it's going to move, it's going to change. But it's not going to change overnight.
"I don't want to be working with somebody who feels they are having to suck it up to be in the same industry as me," he says, evidently disillusioned by the prospect.

McAvoy doesn't necessarily appear frustrated with the film world, but it's clearly a scene he's keen to distance himself from. "It's a status thing; there's so much status in this industry." He goes on, describing a job where an actor was measuring the length of his trailer on set: "and that sounds like bullshit and one of those things that you would only see in a film about Hollywood, but he was measuring his trailer because someone else's trailer was slightly bigger than his."

It's not something I can see McAvoy pandering to. Despite his surroundings, success and substantial salary, his life remains an understated story of pints, Celtic football highs and lows and, well, those burgers. Meanwhile, his tales of onset shenanigans are less about diva demands and more along the lines of hypothetical wheelchair flights with his co-star Patrick Stewart (who also plays a wheelchair-bound Professor X, with a few decades and less hair on him).

"I can do wheelies and 360-spins in my manual wheelchair," his voice loud, thick and fast again. "And then," he continues, with a beaming smile and child-like enthusiasm, "in my motorised wheelchair I can do doughnuts and a forward endo – where you slam on the breaks and travel on the front wheel." (He pauses to demonstrate the motion for me.) "Patrick Stewart's wheelchair's a fucking lorry, it's hard to make that do anything."

"So I don't know who would win a wheelchair fight," McAvoy tells me with a cheeky grin. "But I've got a lot of game."

And there's plenty more of it to come.

Hannah Summers is Square Mile’s contributing editor; read more about her burger chat with McAvoy on her burger (and Bruce Springsteen) blog burgersandbruce.com. X-Men: Apocalypse is out in cinemas on 19 May.