Ambika Mod was just 13 years old when she first read One Day, the bestselling novel by David Nicholls.

But she says her younger self never could’ve imagined that, years later, she would be starring as lead character Emma Morley in the new TV adaptation of the book.

“God no, all of this is still mad to me,” the actress tells BBC News.

Mod, now 29, was born in Hatfield to Indian immigrant parents. Growing up, she says she didn’t see many people from ethnic minority backgrounds taking on lead roles in western productions.

“Even up until my early 20s, it was something that was quite rare in TV and film,” she recalls.

Mod is not wrong. All too often, Black, Asian and other ethnic minority actors have tended to be cast as best friends or supporting characters, rather than in the central roles.

That’s changed in recent years, Mod notes. “But I think we’ve still got a long, long way to go.”

Mod is stepping into a much loved lead role – and one that was previously played by Anne Hathaway in the 2011 film version of One Day.

She stars opposite 27-year-old Leo Woodall, who plays Dexter Mayhew in the new Netflix series.

Both roles were cast regardless of ethnicity.

Nicholls told BBC News that the plan was “always to rewrite the part to fit the best actor and that’s exactly what we did”.

“We thought Ambika had all the qualities of Emma but we didn’t want to ignore her heritage either, so she worked very closely with Nicole [Taylor, series creator] and the writers to make sure that it made sense to her, and I think it worked beautifully,” he added.

Mod agrees, saying: “I think it just opens up a whole other dimension to her character and her identity that is prevalent in the book and I think it becomes even more enhanced by my casting in the series.”

The actress admits it is “significant” to have a non-white person play the role.

“I hope it will open people’s minds a little bit,” she says. “I’m aware how significant it will be to a lot of people, especially young women of colour, young South Asian women in particular.”

One Day is a decades-spanning love story, told in the TV series over 14 episodes. It tells the story of Emma and Dexter’s relationship, revisited on the same day every year over the course of 20 years.

Nicholls said it’s been “thrilling” to have a whole new generation discover a story he wrote 15 years ago.

“It’s amazing that my daughter’s friends seem to be watching it and enjoying it and understanding it even though for them it’s a period drama, it’s like something set in the 1940s,” he said.

“But it’s a thrill. And I suppose the themes of growing up and fear of the future and the choices you make, all of that is universal. So I’m pleased they’re laughing and responding and finding it moving. It’s very, very exciting.”

Neither Mod nor Woodall are newcomers. Mod, who started her career in stand-up comedy, broke through with the BBC’s This Is Going To Hurt, in which she played Shruti, an overwhelmed junior doctor.

Woodall, meanwhile, played Essex boy Jack in The White Lotus – the apparent nephew of Quentin, played by Tom Hollander.

Nonetheless, One Day could be life-changing for both young actors, particularly if it achieves the same level of success as other recent book-to-TV adaptations, such as Normal People and Bridgerton.

Woodall, who read the book during the audition process, admits it was “a bit daunting” to take on such a beloved character.

Dexter is effortlessly charming and comes from a privileged background, but encounters challenges later in life that are explored in the TV series.

“Dexter is kind of tricky to approach and he goes through a lot, and I knew those moments were very important for me to nail,” he says.

“But I was also just really, really excited to do it, and then when I found it Ambika was doing it too, I was very happy.”

Meanwhile, once Mod got the role, she says she spent time working out a back story for Emma, taking in where she came from, her schooling, and other facts to make her “feel more real”.

“For every character I play, I always create a back story,” she said. “I think that’s really important, to know where they came from.”

One Day has received rave reviews, with the Guardian’s Chitra Ramaswamy awarding it five stars and praising it as a “highly bingeable love story packed with magnificent nostalgia and a sublime soundtrack”.

Carol Midgley of the Times also gave it five stars, praising Mod and Woodall’s “fresh, zesty performances”.

“It has taken us all by surprise,” Nicholls said of the positive response.

“I’m hugely grateful to everyone involved to everyone involved for coming up with something so faithful [to the book], but also new and different.”

The pair didn’t have long to build their chemistry before shooting on the series began.

But they found ways to get to know each other on set. One way was by playing games, such as guessing each other’s favourite flavour of crisps.

That helped to stave off any awkwardness when it came to filming sex scenes, Woodall says.

“By the time we were doing intimate scenes,” he continues, “we knew each other so well that we were comfy with each other”.

Published in 2009, David Nicholls’ novel became a word of mouth sensation, sold millions copies around the world and received critical acclaim.

Nicholls, 57, who is also a screenwriter, was an executive producer on the new TV series.

Reflecting on the book’s success, Nicholls wrote recently in the Times: “I’m still very fond of the novel, proud and grateful for its reception.

“I’ve written six novels altogether, but when people occasionally say, ‘I enjoyed your book,’ I know which book they mean. Readers still say that they see themselves in Emma, and see their friends in Dexter, and that means a great deal, that sense of identification.

“And yes,” he added, “it is an epic love story, but revisiting it now it seems to me to be a book primarily about friendship, about our capacity to change each other’s lives for the better through conversation and care”.

Both actors are full of praise for the author, with Woodall describing him as “very humble”.

He was “very involved,” says Mod. “But also even though this was his book and his creation, he was also really happy for everyone to just run with it, and that was really amazing to watch”.

Despite the wild success of the book, the 2011 movie disappointed some readers.

So can the TV series achieve what the film could not?

“I don’t think any of us have any preconceptions or expectations of what might happen,” Mod said.

“But if we do [start getting recognised on the street] it’s only because people will have loved the show and that’s all we want.”

One Day is available to stream on Netflix now.

For more on Ambika Mod’s new role, listen to our interview on BBC Woman’s Hour.