In a tiny, low-lit room underneath the Great Hall of Stirling Castle, Diana Gabaldon is holding court. 'I'll have another Diet Coke, if that's OK?' she asks, and of course it is. She is, after all, the author of Outlander, a series of novels that have had such a significant impact on Scottish culture and tourism, that by now she is literary royalty. That Diet Coke could be served in a crystal-rimmed bejewelled chalice and it would be totally fine.

For those out of the loop, the Outlander series follows the story of Claire Randall, a WWII nurse who finds herself travelling through standing stones and time itself, since she is transported from 1945 to 1743. Claire leaves her husband Frank in the 20th century, and ends up falling in love with an 18th-century highlander by the name of Jamie Fraser. The Jacobite rebellion is brewing, as, it would seem, is plenty of romance and sexual tension. Think plenty of swordplay (and you can take from that what you will).

Claire and Jamie's story is told over eight novels (with another one on the way), and the second series of Starz's television adaptation is currently in production. The series stars Caitriona Balfe as Claire, Sam Heughan as Jamie and Tobias Menzies as Frank (who also plays the baddie of the piece, Black Jack Randall). While the show is only available on Amazon Prime, it still has an enormous cult following, and is responsible for a significant boost in tourism. Historic Scotland for example, reported a 44 per cent surge in summer visitor numbers at Doune Castle, which has a starring role in the show as Castle Leoch (and that's just one example).

People falling over themselves to see some old buildings used in the show is known as the 'Outlander effect', and Diana is no stranger to it. 'It pretty much travels around with me everywhere,' she says. On that note, she tells me about her day, spent filming on location somewhere in Scotland (no, she wouldn't say where). 'Another writer and I went round the corner to the tea shop,' she says. 'We bought a couple of snacks and I gave them a £20 note.' After a fairly routine discussion about limited change and the possibility of opening a tab, the proprietor realised who she was. 'Suddenly, he said, "I've just seen your name on your headphones. I just wanted to tell you that we're giving you everything on the house in gratitude for all the business you've brought us over the last months and years."'

Laughing, she talks about how her husband teases her for her economic contribution to Scotland, but tourism isn't the only thing her writing has helped to generate. 'Anyone who comes with the books seems to experience this flourishing of creativity', she says. 'They want to make t-shirts or pictures that are related to the books or the show, or they want to do videos.

'At one point, some years ago, a nice gentleman had it in mind to do Outlander the musical', she says. 'His idea was to start with a CD of what you call a song cycle, with a dozen high points of the projected show. It turned out very well, though we had to stop doing it when the TV show came along.'

Though Outlander the Musical may not be hitting the West End anytime soon, the hugely popular TV show is a pretty big consolation. Caitriona and Sam have gained a cult following as Claire and Jamie, but does Diana ever feel like there's a crossover between the actors and the characters they portray?

'With Caitriona it's quite separate: there's Caitriona and there's Claire,' she says. 'This is partly because she is physically not at all like Claire. She's a wonderful actress and she personifies Claire very well, but there are elements of Claire that don't translate to the screen.

'People occasionally say that Claire doesn't seem to have a sense of humour on the show and she certainly does in the books, which has nothing to do with the efficiency of the script or Caitriona's really excellent acting, it's to do with the form of Claire's humour, which is based on what she's thinking. You can't get that across without constant voiceover, which would be very irritating. But that means that Caitriona Claire is a separate thing from book Claire.'

This isn't the case with Sam Heughan, who plays red-haired, kilted heartthrob Jamie Fraser, however.

'With Sam, he was the first person cast. They sent me his audition tape and they said, "we think we've found Jamie." This is four days after they started looking. I was amazed, because we thought it would take six months.

'They said they were sending me the audition tapes, but I was on the road, so I googled him on my iPhone as we were driving. At this point he had relatively little film work and just a series of extremely bizarre still photos – his IMDb page is strange. He is a very magnetic, charming young man and a very chameleonic actor, but he's different, physically different, in every part. Some of his photos are strange! I was sitting there typing, "this man is grotesque, what are you thinking?" But that's a running joke with Sam and I, now.'

It didn't take long for the 'grotesque' description to (rightly) fall away, and as fans of the show can testify, it's difficult to imagine that adjective ever being applied to the infamously handsome actor.

'I loaded up the computer not knowing what I was going to see,' she explains, 'and five seconds in I was thinking, "he doesn't look anything like his pictures, he looks great". Five more seconds, and he was gone: it was just Jamie Fraser. He had nailed this particular scene just perfectly. He had the exact right combination of menace, sex, everything.'

But maybe that's what Outlander itself has: a winning combination of menace and sex, plus a good helping of history and romance to boot.

Diana Gabaldon attended an event at Stirling Castle with Neil Oliver as a guest of Book Week Scotland, which runs until Sun 29 Nov. Outlander returns for season 2 in the UK early 2016.