OVER the next hour Richard Rankin will talk astrophysics, biscuits, spoof Twitter accounts, boy band aspirations, swapping IT for acting and the time Elaine C Smith saw him naked. Buckle up …

The Glasgow-born star of hit US television drama Outlander does a rapid-fire line in witty anecdotes. When we meet at the Soho Hotel in London, Rankin is in gregarious mood. There is a mischievous twinkle in his eye and it doesn't take long for us to go roaring off-piste.

First, though, there's his role in Outlander to chat about. Rankin plays Roger Wakefield, an Oxford professor and the adopted son of an Inverness minister, who finds himself unwittingly embroiled in a time-travelling adventure.

Based on the bestselling books of Diana Gabaldon, the show has garnered millions of fans and made its cast into household names around the world.

Outlander centres on former Second World War nurse Claire Randall who, during a second honeymoon to Scotland with her husband Frank in 1945, is transported back to 1743 through a mysterious set of standing stones. It is there, on the brink of the last Jacobite rising, she meets Highlander Jamie Fraser and a love story begins. The third series, shot largely around Scotland, will air on Amazon Prime Video from next Monday.

Rankin joined the cast in late 2015 after almost 18 months of feverish speculation. Other names in the frame to play Roger reportedly included Downton Abbey's Matthew Goode and former Game of Thrones stars Gethin Anthony and Richard Madden.

"Casting was thorough to say the least," says Rankin. "It is the most thorough casting I have done on anything. They had me in time and again with different material and ideas for the character. They were very clear about what they wanted."

From the outset Rankin, 34, was a firm favourite among many Outlander aficionados who lobbied passionately for him to get the role.

"Even from before I was cast I was one of their choices as Roger for a long time," he says. "That was bizarre and put me in a very strange position where I thought: 'I hope the execs on this show don't think I have started my own campaign to be cast …'"

Testament to his heart-throb status is the raft of tribute Twitter accounts that pay homage to everything from the actor's striking blue eyes to his majestic eyebrows.

Are there other, er, parts of his anatomy that have a fan club? "The brain." Wait, I didn't know you had a brain? "Apparently I do and it has got its own Twitter account," he deadpans. "There is the beard – which I'm not sporting today – and other similar ones, quite silly."

The Outlander fans are a pretty great bunch, he says. Even if they do have a habit of trying to fatten him up. According to his Twitter bio, Rankin enjoys a biscuit. "Here's the thing," he interjects. "That's not actually me personally. A lot of people seem to have missed the joke."

Do explain, Richard … "It's the character. Wee Roger from season one gets a biscuit and then asks: 'Can I have another biscuit?'

"I put 'likes a biscuit' on my Twitter bio and now get sent biscuits on an almost daily basis: shortbread, Hobnobs, chocolate Hobnobs, regional biscuits, biscuits from different countries, biscuits I have never heard of …"

Does Rankin eat all these biscuits? "Aye, now and again I'll open a pack or two. But I donate a lot of biscuits to good causes: food banks, people, family, dogs …

"I do like a biscuit, but not quite to the extent that these guys who are running the 'Roger likes a biscuit' campaign think. That was a hashtag on Twitter."

Rankin glances over my shoulder at his publicist. "I don't want to talk too much about biscuits," he laughs. "I can see Jenny looking at me like 'cut the biscuit story short' because that is going to be the headline."

We move swiftly on. Although by the time Rankin stops talking, I have an inkling that poor Jenny may well be wishing he had stuck to biscuits.

The second eldest of four sons, he grew up in Glasgow. His father Colin, now retired, was a police officer while his mother Margaret continues to work in the hotel industry.

Rankin spent his early childhood in the west end, where the family had a flat on Byres Road, before moving to King's Park on the south side (not Rutherglen as Wikipedia states) when he was 10.

His parents had their work cut out. "It was chaos with four boys in the house," he says. "I was always running away when I was younger. I was brought home by my dad's colleagues a couple of times for running off on mad adventures."

Where did he go? "About 300 yards. I used to run away from home because that seemed a very romantic idea. I don't even think I was particularly running away from anything. I was just escaping."

Once Rankin was found hiding under a parked car in a nearby lane. Another time he made a break for freedom from his primary-school classroom. "I had two teachers running after me," he says. "Again, I didn't get very far. I tripped up and hurt my knee quite badly; it was all grazed and bloody."

As we probe deeper into his past, there are moments where it feels like I'm playing amateur psychologist. Why did he run away? Was it about attention? Did he do it because he was bored?

"I think I was just adventurous," he shrugs. "Whenever there was any scaffolding up you could often find me at the top of it – or on tenement roofs. I would climb anything."

He's not kidding. While trying to avoid a telling off from his mother for some escapade or another, a young Rankin produced a manoeuvre that Spider-Man would be hard-pressed to follow.

"The walls in the hall of our flat were quite close together," he explains. "When I was younger I used to be able to put my hands and feet on each side and climb up."

Having shimmied to the ceiling, Rankin remained there silently poised. Well, almost silently. "My mum was wandering up and down the hall looking for me and I couldn't help but snigger. When she looked up, her face had this amazing mixed emotion of utter fury and absolute hilarity."

He has loads of stories in this vein, although Rankin is adamant that there was no malevolent or troubling underlying reason for his high jinks. "It wasn't because family life was bad or anything," he asserts. "I was a handful. I was utter mischief when I was younger."

Adventure was a theme that loomed large. And the more thrilling the better. "Anything dangerous seemed to attract my attention," he says.

Rankin was grounded for an entire month on one occasion. What had he done to warrant that? "I was suspended from school." He glances at his publicist again. "Jenny is hating this interview."

He turns back to me. "I broke a window. There was a wee boy on the other side of it annoying me, so I punched the window a couple of times and it shattered. I was mortified. I thought: 'Oh no, I'm really in trouble' because I never got in trouble at school. I knew that was bad.

"I wasn't suspended for long. I think it might even have been the Friday and then back at school on the Monday. This was in primary seven."

While Rankin wouldn't begin acting until his twenties, there were signs of a budding performer long before that. "I went through many a creative outlet," he says. "My dad was a drummer and into his music. That was something he had pursued early on before he joined the police.

"My earliest memory of music was listening to Billy Joel. I have many vivid memories of me dancing around in a nappy singing Uptown Girl."

Sadly, there is little hope of this turning up in the vaults of Before They Were Stars. "There is no footage that exists," he says, grinning. "A video camera back then was a luxury."

This was the 1980s when, unlike today's smartphone era, camcorders were for high days and holidays. Which is perhaps as well given his penchant for homemade skits and shows.

Rankin went through a phase of pretending to be RoboCop, while other gems included doing a Michael Jackson impersonation concert in the school playground that resulted in spilled blood (his own) and abject humiliation (also his).

"I was charging £2 a ticket, which I thought was reasonable," he recalls. "Although I may have dropped it to £1, I can't remember the economics of it.

"I had bought smoke bombs. I'm not quite sure how I thought logistically I was going to have all these pyrotechnics going off. But obviously in a young boy's head there is lights, smoke and a full concert atmosphere.

"I managed to sell three tickets. It turned out an utter disaster and I split my head open trying to do a backflip off a pillar as my finale. The smoke bombs didn't work either."

Throughout his teens Rankin continued to daydream about stardom. "I wanted to be a writer, a singer and there was a very brief embarrassing period where I wanted to be in a boy band," he says. "I even had my hair floppy and Nick Carter-esque."

Rankin began a course in information technology at Glasgow Caledonian University, but a chance conversation while on holiday to Los Angeles would change his life path.

"I met a producer – I think he worked on The OC, although I'm not 100 per cent sure – but he was one of the execs on a show like that," says Rankin. "He was at the Roosevelt Hotel on Hollywood Boulevard with his cast and they were chilling out."

The pair got chatting and the producer told Rankin he had a good look for an actor and asked if he had ever considered it as a career? "I was like: 'OK, whatever …' but it was enough to plant the seed."

After returning to Glasgow, Rankin auditioned for a place to study acting at Langside College. Back then he was Richard Harris, but adopted his mother's maiden name when applying for his Equity card to avoid confusion with the late Irish actor.

Rankin's early roles included the now defunct BBC sitcom Legit and meeting a grisly end on Taggart – a rite of passage for any aspiring Scottish actor – before he joined the cast of cult comedy sketch show Burnistoun in 2009.

He went on to tour with the National Theatre of Scotland's globally-acclaimed production of Gregory Burke's Black Watch in 2010. Even now Rankin gets misty-eyed when talking about it.

"I was in it for three years and that was one of the best experiences of my life," he says. "I don't think I will ever beat that in terms of a theatrical production. I still dream about that show."

Parts in BBC war drama The Crimson Field and as a lottery winner in Kay Mellor's The Syndicate followed. Rankin starred opposite Anne-Marie Duff in psychological crime thriller From Darkness, and more recently alongside Morven Christie and Vicky McClure in The Replacement.

Like fellow Scot Sam Heughan, who plays Jamie Fraser in Outlander, Rankin has found himself in a position where his fame has exploded internationally yet in his homeland he remains better known for other TV roles.

That may be about to change. It has long been a gripe that Scottish viewers have been unable to see Outlander on terrestrial telly, but in June it was announced that More4 had bought the rights to screen the first series from Sony Pictures Television.

Indeed, we should get to know his on-screen alter ego pretty well. "If we are talking about the books, I happen to know that Roger is very much still there in book nine," he says, referring to the latest Outlander novel that Gabaldon is writing.

"Diana does this thing where she releases little snippets, so there have been scenes that Roger has been involved in. Should we run in tandem with the books, I will be there in season nine."

Away from work, Rankin can often be found listening to podcasts by astrophysicist Neil deGrasse Tyson or tinkering with computers.

"I have always had an interest in IT and I still do," he says. "When I was 15 or 16 I built my dad's computer. If I wasn't an actor I would be doing that or something in the sciences."

The only time he clams up is when asked about relationships. "No comment," is the polite but firm response. Many Outlander fans would love to see Rankin romantically involved with his co-star Sophie Skelton, whose character Brianna Randall falls in love with bookish Roger.

"They do hint at it any time they see photos of Sophie and I," he says. "'Aww, cute couple', 'You look great together', 'Oh, it's love …' and all this carry-on.

"I know that Sam and Caitriona [Balfe] have gone through a fair amount of drama with people flat-out insisting they are a couple. They present to the public their respective partners, but the [diehard fans] go: 'Nope, it's all a lie. A facade. You are a couple,' because folk are so invested in it."

Speaking of rumours, is it true Elaine C Smith has seen him naked? "Yes, I suppose she has," he muses. It transpires Rankin got his kit off when starring alongside Blythe Duff in David Harrower's Good with People at the Traverse Theatre in Edinburgh in 2012.

"There was also a show – and I think this might be the one Elaine was talking about – that I did with Gabriel Quigley, a David Ireland play called Most Favoured [as part of A Play, A Pie and A Pint in 2013] where I had to get down to my pants, so I was pretty much naked.

"Any time I've ever seen Elaine at anything it is the first thing she will say to whoever she is with. She was with Barbara Rafferty and said: 'I've seen him naked …' It is quite a funny wee anecdote."

What else does he have in the pipeline? "Outlander …" Rankin bursts into raucous laughter. "I'm going to be on that for a long time. Season four films over nine or 10 months, so I don't think I have much time to do anything else."

Long term, however, Rankin has plenty of aspirations. "The big things for me, the ones to tick off my bucket list, are going to be Hamlet and Macbeth. I would love to do a bit of Shakespeare and tackle Macbeth and at some point have the chops to take on Hamlet."

Rankin no longer has to go running off in search of adventure. He's already living it.

Outlander series three is available on Amazon Prime Video from September 11. With thanks to the Soho Hotel in London (firmdalehotels.com) and Paul Smith (paulsmith.com)