Making movies based on books is hardly a new phenomenon.
But when the novel in question has sold millions of copies worldwide, been translated into 40 different languages and received critical acclaim, getting the film version right is crucial.
The romantic story of Emma and Dexter's friendship revisited on the same day every year over the course of two decades, is played by Anne Hathaway and Jim Sturgess on the big screen.
"I didn't realise how deeply loved the book was when I signed on to play Emma and I sort of wish I didn't know," Hathaway admits.
"The weight of expectation got even larger, and I quickly realised I was going to disappoint a lot of people.Much has been made of Anne Hathaway's British accent in the film
"People have their own idea as to who these characters are, how they act, how they sound."
But the 28-year-old, who has also starred in The Devil Wears Prada and Alice In Wonderland, says she thinks films can sometimes enhance the book it is being based on.
"Sometimes there are things you can do on screen that you can't necessarily do in a book and there are often times a book will be wonderful in a different way to the movie."
But for Sturgess, whose most notable film to date was the 2008 movie 21, the stress of playing a well-loved character came much later.
"I was blissfully unaware," he laughs.
"As I'd been out of the country I wasn't really aware of the gravity of this book.
"It got more daunting as the film went on and when we were shooting in Paris and London I would see this orange book cover pop up everywhere. And then I started to get a little nervous, but I was just excited to be in a film that people might actually go and see."
The movie has certainly helped boost sales of the book in the UK.
According to trade website The Bookseller, more than 60,000 copies of One Day were shifted across the UK last week and two years since it was first published, the novel is once again top of the fiction best-seller list.
Author David Nicholls started his career as a screenwriter and admits he understood some of the issues that surrounded bringing a book to life on the big screen.Continue reading the main story Colin Paterson BBC entertainment reporter
One Day has a running time of one hour 47 minutes and 52 seconds. The story spans 20 years. This means that each 15 July is granted on average five minutes and 23 seconds of screen time.
At Tuesday's premiere Anne Hathaway explained to me how the book's author David Nicholls managed to condense 437 pages into a celluloid-sized chunk. "Some years got more attention than others. One year the shot is of me swimming and you hope that communicates all the angst and growth that Emma went through in that year. Ha ha."
The year solely covered by the breaststroke is 1997. In the book that chapter has Dexter being sacked from a TV show, while Emma meets a publisher, ends an affair (that never even starts in the film) and resigns from her job. That is a lot to covey in one length of a swimming pool.
One Day - one almighty rush."I was aware that we'd have to lose things that I loved and there were whole elements of the book that just wouldn't work on screen."
The writer, who also penned the 2003 book Starter for Ten and The Understudy in 2005, wrote the screenplay for the film, which he says was the hardest script he has ever worked on.
"Inevitably, as the author you feel possessive and you want to keep hold of it.
"You are always concerned about whether people who loved the book would also love the film, but they did and that was incredibly gratifying."
Despite the writer's optimism, the film took a disappointing $5.6m (£3.4m) in the US.
And reviews for the film in the UK have been mixed, with many critics claiming the movie sticks a little too rigidly to the book.
The Telegraph's Sukhdev Sandhu says although the movie "keeps almost everything Nicholl's original novel contained" it lacked a "huge emotional punch".
Emma Dibdin's Total Film review says: "Much of Nicholls' sharp dialogue and emotional truth remains intact, but the over-faithful translation does little to compensate for what can't be lifted from the page."
But Nicholls shrugs off the criticism, insisting the "worst thing" one can do is "watch the movie and turn the pages in your mind".
He insists the audience must "experience the film as it's own thing".
One element that may have baffled some fans, was why director Lone Scherfig cast an American actress for the role of northerner Emma.One Day author David Nicholls also wrote the film's screenplay
"I went for someone who is very warm, very experienced, and who is a strong dramatic actress and I think the way she has interpreted Emma makes a lot of sense," Scherfig explains.
Much has been made of Hathaway's accent in the film, which appears stronger in some scenes than others.
And at the UK film premiere on Tuesday in London, Scherfig admits the accent had been a "concern" of hers, adding: "But they're actors and actors always play things that they are not, that's part of their job."
Nicholls agrees, saying he too understood the "trepidation" but insists, it was not an issue to him seeing Hathaway convey his Emma.
"British actors are constantly playing American roles and it's always seen as a rather good thing, and I see no reason why it shouldn't be a good thing working the other way."