It must be difficult to get entirely swept up in the magic of the movies when you are the man who once changed Pennywise’s nappies. This is the strange position that actor Stellan Skarsgård finds himself in, as he promotes his new film, Borg vs McEnroe, while his 27-year-old son, Bill Skarsgård, is receiving rave reviews for playing the demonic clown in a new adaptation of Stephen King’s It while his eldest son, Alexander, is about to win an Emmy for his role in Big Little Lies. “I was happy when he was doing It because he had so much fun, and that’s where the joy was really,” says Skarsgård senior, frowning thoughtfully out of the hotel room window, as if searching for the right words amid the rooftop air vents. “It’s also kind of ridiculous, all of it, isn’t it? On Sunday, Alexander goes up for the Emmy … It’s kind of silly, isn’t it?”

Silly, perhaps, but also endlessly interesting. The nature of success and how it impacts our relationships – specifically “the sort of loneliness in the midst of your being the most famous person in the world” – was what first attracted Skarsgård to Borg vs McEnroe; directed by Danish documentary-turned-feature director Janus Metz Pedersen, it tells the story of the rivalry between Skarsgård’s Swedish countryman Björn Borg (Sverrir Gudnason) and the US’s John McEnroe (Shia LaBeouf), and culminates in a nail-biting showdown at the 1980 Wimbledon final.

Skarsgård, who says he has zero interest in sport of any kind, says even he can remember this particular match. “At that time in Sweden we had two television channels, there was no internet, everybody watched the same things, which meant it was a pretty bizarre, welfare, social-democratic monoculture. Borg was at the very centre of that for many years.”

It wasn’t just in Sweden either. International interest in the rivalry was captured by the two players’ contrasting characters. McEnroe was the hotheaded young pretender, whose eagerness to unseat the champion resulted in mid-set tantrums. Borg was a cool-headed champion; the ice to McEnroe’s fire, the gentleman to his rebel. Skarsgård plays the man credited with creating this ruthless focus, Borg’s long-term coach Lennart Bergelin.

The film, which, in the Nordic region, is simply titled Borg, could perhaps be accused of taking sides, but Skarsgård finds it harder to declare himself for either Team Rebel or Team Gentleman. “I like to see myself as both. I do not always accept the conventions or the rules of society, but I’m trying to be nice to everybody. I don’t want to hurt anybody.” A gentleman rebel, then? “Yes, sort of.”

Skarsgård as Lennart Bergelin in Borg vs McEnroe. Photograph: Julie Vrabelova/Artificial Eye

As a past-his-prime Bergelin, Skarsgård provides the film with a compelling emotional centre, but he insists the competitive father-son dynamic is in no way drawn from his own relationships with his acting offspring [four of his eight children are actors]. “No, because I don’t coach. I haven’t helped them at all, I haven’t introduced them to agents or opened any doors for them.” With typically mischievous modesty, he is also keen to give the prosthetics their fair share of the credit. “First thing, I put on a bald cap and a combover and then I was pretty pleased with myself. I thought: ‘Well that’s him’, so now I don’t have to deal with the externality any more. Now, I can concentrate on my job.”

That job is the same one Skarsgård has been doing for half a century now, ever since, aged 16, he became a Swedish TV star playing a lovable rascal in a show called Bombi Bitt Och Jag. After several years on stage and screen in his home country, he made the move to Hollywood, with notable roles in The Hunt for Red October (1990), Good Will Hunting (1997), Mamma Mia (2008) and as the Thor-aiding astrophysicist Erik Selvig in the Marvel universe.

The culture shock continues to this day, he says. “The first time I worked in America, in one month I got more praise from the director than I had in 10 years in Sweden. That was not because I was 10 times as good.” Perhaps because of such irresolvable differences, he has always kept one of his size 12 feet in the world of European cinema, making several films with his two favourite collaborator-playmates, Norwegian director Hans Petter Moland (In Order of Disappearance and the upcoming Out Stealing Horses) and infamous provocateur Lars von Trier, with whom he has worked six times.

If Skarsgård does allow any fatherly feelings into his professional life, they are reserved for his directors, whom he regards with the amused benevolence of an indulgent parent. “Most good directors, I have this feeling, had something weird about their childhood,” he says with a kind smile. “They had very little social contact when they were small. They were all sitting in their room creating fantasy worlds, and they don’t want to abandon that control. They’re all control freaks, one way or another.”

Stellan Skarsgard with sons Gustaf, left and Bill. Photograph: IBL/Rex/Shutterstock

And when he isn’t indulging creative minds on set? “Well, I’ve been changing diapers for 40 years,” he says. Skarsgård’s children range in age from 41 to five, and include five sons and a daughter from his first marriage, plus two sons from his current one. He sees most of them every week, provided someone is not off shooting somewhere exotic. “I mean, even if I’ve worked a lot, for around eight months a year, I’ve been at home, taking care of kids and cooking and stuff,” he says. “I cook at lot. I really love that, and it’s a very good way to make sure that your kids come home. If you cook well, they’ll come back.”

Though Skarsgård is proud to typify the gender-equal Swedish ideal of co-parenting, he is even less interested in flag-waving than he is in tennis, which helps explain why Borg vs McEnroe is only his second Swedish film in 20 years. “I really despise nationalism, it’s such stupidity; but I succumb. When Sweden plays football in the World Cup – if we ever get there – then, I succumb to that. I’ve been to some big matches and it is exciting and everybody’s screaming and everything, and I think: ‘Nuremberg rally’, y’know?”

Yet, for all his antipathy to the very notion of national heroes, there is something about Borg he finds utterly fascinating. It was while shooting a scene with Leo Borg, the tennis player’s real-life 14-year-old son, who plays him as a child in the film, that Skarsgård worked out what that is. “He’s a very good tennis player, but he’s never been in front of a camera before and I was thinking: ‘What is it that makes him so good at acting?’ It’s because the focus that you have to have to be a good tennis player is the same focus you have to have to be a good actor. You see in his eyes that a lot of things are burning inside his head, even if he doesn’t express anything. Film acting is very much about that.”

Borg vs McEnroe at Toronto film festival: Shia LaBeouf (McEnroe), director Janus Metz Pedersen, Sverrir Gudnason (Borg) and Skarsgård (Bergelin). Photograph: Canadian Press/Rex/Shutterstock

He has been even more impressed by the “beautiful” talent of Shia LaBeouf, with whom he also co-starred in Von Trier’s Nymphomaniac, despite never having shared a scene. He disagrees that LaBeouf’s bad boy rep makes him an obvious choice to play the young McEnroe. “I totally understand the choice, but I think it’s right not because of Shia’s personality, but because of his skills … I mean he has had problems when he gets drunk, probably he can’t handle it or whatever,” he shrugs. “And that has to do with his childhood and the things that he hasn’t solved in his life. It shouldn’t cast any shadow over him as an artist. There are some artists that are assholes but are straight all the time and I despise them.”

Ultimately, it’s the joy of working with interesting non-assholes that has motivated Skarsgård’s long, varied career. “Those actors make me do something I could never have invented myself,” he says. It’s what has kept him invested on set and even now, still passionately rooting for a film about a subject in which he has no particular interest. That, and the sheer relief of removing your prosthetics at the end of a long shoot. “Just sitting on the stands there with a bald cap filling with sweat. It was disgusting … I was so happy on the last day.”

Borg vs McEnroe is released on Friday