It became a best-seller and spawned a hit movie, but The Guernsey Literary & Potato Peel Pie Society was never supposed to be Annie Barrows’ book.

The story was actually the brainchild of her aunt, Mary Ann Shaffer, an aspiring author, who dreamt up the story while stuck at Guernsey airport back in 1976.

According to Annie, her aunt then spent the next 20 years researching the occupation of the Channel Islands during World War II, before ever putting anything down on paper.

“Her writing group finally threatened to lock her up in a room with just a typewriter if she didn’t start writing. So, she sat down and started writing,” Annie, 56, tells Mamamia.

“She completed the manuscript, and she found a literary agent, and the agent found a publisher, and the book was going to be published. This was so incredibly thrilling, because Mary Ann had always wanted to write a book that somebody would like enough to want to publish. That was her life dream.”

Annie said the whole family was "thrilled" for her, but their happiness soon turned to devastation.

"Then she got sick. It was so sad," she says.

Shortly after falling ill, Mary Ann's editor handed her back a huge stack of rewrites, which Annie describes as being quite "serious". Not in any condition to do the work, the book got pushed back further and further.

"She was so ill. She called me up and said, 'You're the other writer in the family, can you finish this?' I have a career as a children's book writer," she explains.

Although Annie immediately said yes, she was quietly freaking out about just how mammoth the task at hand was.

"I was thinking, 'Oh my God, this is so impossible. This is ridiculous.' But I was never going to say no, because it was Mary Ann, and it was her life dream," she says.

Daunted by the project, Annie says she kept putting it off, until finally, the words just flowed out of her.

"I basically waited as long as I could, and then I couldn't wait anymore. I sat down to do it, and... that's when I realised, this is just going to be the most beautiful project," she says.

Annie says she comes from a family of storytellers, saying, "nobody gives you a short answer to anything", so she could hear Mary Ann's voice loud and clear as she finished the story.

"My lifetime of listening to her came out," she says.

"I was like, 'Oh, of course. I know how she would write this sentence. I know what this should sound like.' It was such a beautiful, fulfilling writing experience, because it was as if I was hearing her speak, and I would just keep writing and keep writing...

"Her voice was so profoundly entertaining and witty and charming, and I knew exactly how it would go. It was like listening to a song you know. So that's how I finished the book."

Even though she could hear her aunt's voice in her head, Annie says she still worried about whether or not Mary Ann would approve.

"I was tearing my hair out at certain points, thinking, 'Is this okay?'" she adds.

"I had one conversation with her before I began, and we went over everything. And then she retreated into that place that very sick people go. She didn't want to talk about it. She was just like, 'To heck with the book! What's the book? I don't care.' I just didn't know sometimes what I should do."

She says while she hopes she did an "honourable job" of finishing the story, it's still something that worries her.

"But I'm comforted by the fact that my mother, who is her sister, and my cousin, who is her daughter, can't tell which stuff is mine and which stuff is Mary Ann's," she adds.

And even though the book is now out in the world, Annie says she still asks herself "all the time" what her aunt would have thought of it.

"I hope she would like it. I think she would. I hope she would. I don't know!" she says with a laugh.

Annie says even though it was ultimately her who finished telling the story, so much of her aunt remains in the book.

"Juliet's personality is like my aunt's. Juliet's willingness to be fascinated by somebody else's story is an essential part of her character, that is not just curiosity, but a delighted generosity of interest," she says.

"That's 100 percent Mary Ann."

When asked about the massive response the book has received, Annie is still disbelieving, and can't help but laugh.

"I thought it would do well and people would appreciate it. But I thought the most that was going to happen was I would finish my work and Mary Ann would be so glad her book was being published, and we would all just be happy, and then I would go back to my regular life," she says.

Clearly, it was a little more complicated than that though.

"I found myself being the only author on the ground with a major international best-seller hit. I didn't even know things like this happened... I had no idea any of this stuff would happen," she says incredulously.

"A few years later, I finally saw my family again, and we got back to normal... But my life was changed."

She also never actually believed the book would be turned into a movie, thinking they were only "joking" by even suggesting it.

"Before the book even came out, the producers, they optioned the book. They took me out to lunch and told me they wanted to make a movie, and I was like, 'Oh, you guys are so great! I really like you. Sure, go ahead!' But I was thinking, 'That will never happen,'" she laughs.

"And, by God, 10 years later, due to their perseverance, it happened."

Annie says she decided to keep her distance from the making of the movie, because "the streets of Hollywood are paved with the scripts of writers who demanded to be part of the process".

"I thought the most helpful thing I can do to actually make this movie happen is bow out and leave it to people who know what they're doing," she says.

But she did ask to be invited to the premiere.

Still, even with all of the wheels in motion, Annie says she never actually thought the film would go ahead.

"It wasn't until last spring, when they were filming, that I read the final script and saw what they were doing, and saw them filming the scene when Juliet comes to the island and eats potato peel pie for the first time. That's when I really started to think, 'My God, this is happening,'" she says.

And while she loved the movie, Annie admits she does have one regret - that her aunt never got to see it.

"She would have loved this movie," she says.

She also admits the actors playing the characters were "all much better looking than I had ever imagined".

"You always picture your characters as some sort of amalgam of people you see. I always thought Juliet was going to be pretty, but Lily James is really, really pretty," she says.

"They were a more glamorous version of what I had imagined, let's put it like that."

The one consistency with this book is that it never stops surprising Annie.

"Over and over again with this book, I've said, 'Oh, that'll never happen,' and it just has this incredible buoyant success, where every time I doubt it, it just surmounts my doubt," she adds.

LISTEN: Morris Gleitzman on getting kids to read. Post continues below.

Even though her aunt is no longer around to see how it's all played out, Annie says she's convinced there's been some divine intervention on her behalf.

"I have thought that in my life, that this is somehow being directed from above," she says.

"I really hope that's the case, because at least then, somebody knows what the hell is going on and is in charge. Because, clearly, I am way behind... Each thing stuns me. Each time this book reaches a new height, I'm completely stunned and astonished."

And while fans of the book would undoubtedly love the story to continue, Annie says there won't be a sequel - she's happy to leave The Guernsey Literary & Potato Peel Pie Society exactly where it is.

"It's very much Mary Ann's creation... I know everybody wants more, but it's perfect the way it is. I'm not the person who can give them more, I don't think," she says

"It's Mary Ann's. It started with Mary Ann, and that's how I always think of it."