Image copyright Simon & Schuster
Anna Todd's book After has sold 11 million copies worldwide
At 24 years old, Anna Todd felt her life had come to a bit of a standstill.
"I was a struggling army wife going to community college who didn't know what to do with my life," she tells the BBC.
"I would read a lot of fan fiction on [publishing website] Wattpad and felt I had read everything so one day I decided to write a chapter."
That chapter, the start of a story based on One Direction singer Harry Styles, would eventually be turned into a book series and now a Hollywood film.
Now 29, the Ohio native has published five books in the After series and three other novels.
'I couldn't even use my own name on Twitter'
The story of a relationship between college student Tessa Young and Harry Styles (who would become Hardin Scott in print) was Anna's passion project as a huge One Direction fan.
"When I started I had no followers but I did it for fun," Todd says.
"After I wrote the first chapter I wanted to know what happened so every day I would write a chapter and it became more popular.
"I could see the numbers go up and when I tried to make a Twitter account I couldn't even use my own name because all these fan accounts already existedI realised people actually cared and loved it.
"Wattpad contacted me and said publishers and agents had reached out, I was even getting messages from agents and didn't understand what was happening.
"I went to New York and met with four publishing houses and there was an auctionevery publisher wanted me to take down my online chapters and I didn't want to do it.
"I kept saying no and they gave in."
Todd wanted to keep her work online so that new people found it and also so she could read comments.
To date, her online version has received over 1.5 billion views, whilst her published book has sold over 11 million copies worldwide and has been translated into 30 languages.
After: The Movie
Todd is three weeks into her role as producer on After: The Movie and speaks to the BBC from the set, which is based in Atlanta, Georgia.
"Being here feels amazing and really surreal, my entire life has changed. It's crazy seeing these characters come to life and I've been very involved in the re-writes and casting."
The lead, Hardin Scott is played by Hero Fiennes Tiffin, who is best known for playing young Tom Riddle in Harry Potter and the Half Blood Prince.
He is also the nephew of actors Ralph and Joseph Fiennes.
His co-lead, Josephine Langford, who also has famous connections as the sister of 13 Reasons Why actress Katherine, plays Tessa Young.