The Big Bang Theory is the most watched TV show in the world. Its undisputed star is Jim Parsons, who plays brilliant – and brilliantly exasperating – know-it-all theoretical physicist Sheldon Cooper.

His performance has already won him four Emmys and a Golden Globe and turned an old-fashioned sitcom set-up with a simple ‘beauty and the geeks’ premise into a global smash – attracting an audience of over 23 million in the US, almost 10 million more than second place Modern Family. It is easy to think more people in America watch The Big Bang Theory than actually believe in the big bang theory.

“I am going to have to say more people believe in the big bang theory than watch The Big Bang Theory, as successful as the show is,” says Parsons. “And I’ll tell you why. I think denying evolution is a very colourful story, which is more fun to report on, but I do believe… Oh hell, what do I know? I live in LA and New York.”

I feel a severe sense of gratitude toward the TV show

The Big Issue is meeting Jim Parsons in a boutique hotel in central London. The youthful 41-year-old Parsons is a warmer, less abrasive version of the character that has made him one of the most recognisable faces in the world. He speaks with the same distinctive rhythm and soft Texan drawl as his onscreen alter-ego but has ditched Sheldon’s trademark T-shirt over sweatshirt combo for a sharp, dark navy suit with a skinny tie and polished shoes as shiny and bright as his disposition.

Part of what makes Parsons’ explosion on to the A-list so welcome is that he spent years as a jobbing actor with the now defunct Houston-based group Infernal Bridegroom Productions, staging everything from Othello to Guys and Dolls, with occasional minor TV and film roles, including a bit part in Zach Braff’s Garden State. Was the dream always to be cast in a long-running TV phenomenon?

“No, not that I thought it wasn’t either,” he says. “I just felt there are many, many ways of living this dream. I wanted to make my living as an actor and whatever medium that took I was very fine with. I’m quite pleased with how things worked out, obviously.”

Parsons and fellow principles Johnny Galecki (Leonard) and Kaley Cuoco-Sweeting (Penny) earn $1m per 22-minute episode and have secured more than one per cent of syndication payments, estimated to generate them $100m each as the show lives on eternally in syndication.

Indeed it already airs constantly on some TV channels and has helped the drive to make geek chic. Since it started in 2007, it has provided role models for brainy sci-fi fans who struggle when it comes to the opposite sex. By definition, geek culture exists outside the mainstream. By popularising that lifestyle, has the show destroyed geekdom as a result?

“God I hope not,” he says with concern in his voice. “Geek culture, like anything else, is an evolving thing. It is interesting to see a lot of things that were/are termed geek culture are now mainstream but that very thing leaves an empty space to be filled and therefore there’s something else that’s becoming geek culture as we speak. But it’s also a way of being, a state of mind.”

Parsons listens to questions attentively, perched on the edge of his chair. His eyes either explore the room, as if he were addressing a large crowd, or stare directly and intensely. There are traces of tics and mannerisms that are magnified tenfold in his performance as Sheldon. As socially awkward as he is scientifically gifted, Sheldon certainly sits somewhere on the spectrum with his quirky obsessive-compulsive tendencies and has become a touchstone for teens, geeks and pretty much anyone of any age who feels like an outsider.

I was voted class friendliest at school! I wonder if I’m still as friendly? I probably am

The drawback of creating such a widely loved character and being so convincing in the role is that you are always going to be inescapably associated with it. Daniel Radcliffe might always feel trapped as Harry Potter – is it different for Parsons, an actor who was in the business for a long time before receiving his huge break?

“It is a major difference, for me at least. I feel a severe sense of gratitude toward the TV show. It doesn’t overly concern me either way because it’s led me to such a happy, happy place in my life. I’m pretty at peace with whatever happens. I get to do what I love and I have people around me who love me.

“I feel so happy,” he repeats. “I hope that doesn’t sound irritating.”

Parsons’ life is deliciously boring. He has been in a relationship with art director Todd Spiewak for more than a decade, he loves to follow the tennis circuit, is a fan of the Williams sisters and Andy Murray (“though I would sure like to watch Roger Federer win another major”) but tends to have a fixed bedtime of nine o’clock every night.

“I’m a lot warmer and better at reading signals from other human beings than he is,” he says. Parsons never refers to Sheldon by name. “If no one’s ever met me and they’ve seen me a lot on the TV show, I don’t see how they couldn’t come in assuming I’m going to be awkward or disconnected in that way. People don’t think I’m going to answer science questions but I sometimes think they don’t know exactly how approachable I’ll be.”

That is probably not a terrible thing. “No, probably not,” he agrees. “It’s better to surprise people with your warmth than have them running up and embracing you. I don’t wake up ready to embrace everybody every morning. It takes some coffee and it takes some thought.

“I should say first I find that, knock on wood, I have the most pleasant and wonderful interactions with strangers because of the show,” he says, tapping the protective glass cover of the coffee table. “But there are times when you notice people looking at you and even if you consciously know why, you do feel a little alien. It can be very disconcerting.”

He’s now playing Oh in the DreamWorks-animated film Home, about a rather benign race of aliens called the Boov who invade Earth after their own planet is destroyed. All humans are forcibly relocated to quite plush refugee camps in Australia while the Boov move into the rest of the planet’s towns and cities.

Alongside the primary colours and slapstick humour, the film is thick in the subtext about immigration and prejudice, told through the unlikely relationship Oh develops with a young lost girl, Gratuity ‘Tip’ Tucci, voiced by superstar songstress Rihanna. While love stories tend to be the staple of film plots, is friendship just as important?

“I was voted class friendliest at school! I wonder if I’m still as friendly? I probably am,” he grins and lets out a sudden howl of laughter. “I think in many ways friendship is much more important. It’s hard to find true love unless you come from a place of loving yourself. Tied into that is confidence. Friendship and feeling you belong is the base from which loving yourself and feeling confident grows.

“As a performer there’s great joy in playing someone or something who doesn’t understand what’s around him. It’s that childlike, ‘Everything’s new! EVERYthing’s NEW!’ It’s funny how it changes your view on things, you feel more wide-eyed. I guess it’s contagious. And as funny and beautiful as the movie is, my favourite part is that central message of how things can go right when we don’t approach something or someone with preconceived notions. I don’t know, it feels like a message that never gets old and in today’s world is all the more important.”

Parsons had no preconceptions when it came to Rihanna. “I’ve learned well enough in this business, you never know what you’re going to get. She’s as famous to me as she is to anybody else on the street. I don’t know if she felt the same way about me. For all I know she’d never seen me before.

“We are obviously very different but she has a quick wit and is very warm. I think that comes across in her music. I especially like that song, what’s it called…?” He sings a few lines of Only Girl (In The World). “That is kind of an odd song for me to dance to but it does make me want to move.”

But it is another singer Parsons is hopelessly devoted to. The mention of Olivia Newton-John makes him light up. “Yes! I like almost all of her songs. I like… I’m not going to be good with titles now that we’re talking about this. Hold on, it’s right here on my iPod.” He whips out a music player from his pocket and scrolls through the menu.

“O for Olivia… there she is! My favourite ones: A Little More Love, Make a Move on Me and Please Mr Please,” he lists with Sheldon-like conciseness. “I play these over and over. Now I also really like Magic, Suddenly and Heart Attack but…”

Parsons presses play and the funky opening riffs of A Little More Love fill the room. He sways in his seat and swishes a jazz hand at the first spiky guitar.

“Who doesn’t like that?” he smiles. I confess that my knowledge of her back catalogue does not extend beyond the Grease soundtrack, Physical and Xanadu.

“Really?! You really need to go on a little trip down Olivia Newton-John Lane. When she switched over and became a little more rock after her mild experiment in country she really shone. See, I don’t have Physical. I loved the song when I was young but I think I wore that out. It’s lost its magic for me.”

(Following our interview, I contacted Newton-John’s ‘people’ who were delighted that Parsons was a fan and invited him to one of her Las Vegas concerts.)

Later in the summer Parsons will begin filming The Big Bang Theory’s ninth season, with a 10th already confirmed and the door left open for more. Before that he will appear on Broadway playing the only person in the universe more of an annoying know-it-all than Sheldon – God. An Act of God is a comedy by The Daily Show’s former chief writer David Javerbaum and promises to answer all of our existential questions. Has it helped answer his own?

“No. It’s probably opened up more existential questions, actually. I remember being very young and confused about religion and life in general. I thought you’d get older and get more answers and a teacher said, ‘Things only gets more confusing the longer you live’. She was right. Now it doesn’t bother me. There are just a lot of questions, more questions. I have no theory of everything and I kind of like it.”

Jim Parsons

Jim Parsons: “It’s hard to find true love unless you love yourself”