“Celaena walked into my head the second I asked myself, ‘What if Cinderella was an assassin?’ I just wanted to know more and more about her,” says 27-year-old author Sarah J. Maas of the charismatic teen assassin who debuted in Throne of Glass and now returns in Maas’ highly anticipated second book, Crown of Midnight. Though Maas describes Celaena’s arrival as “an instant lightning-bolt of inspiration,” publishing Throne of Glass took 10 years—a fact that’s often lost in the buzz of her recent success.

“I started writing this series when I was 16; I wrote the first three books in the series over the course of six years, she says. Then I revised and rewrote every word of it. As a teenager—well, this was my first time writing, and it really showed. But I kept at it, and eventually, when I saw Throne of Glass on the bookshelf—there was my outrageous, wild dream come true.” It’s clear that by now, Maas has marshaled her talents: In a starred review, Kirkus calls Celaena “vivid…loving and brutally violent in turn…a fully realized heroine,” and Maas acknowledges her as a forceful presence shaping the storyline. In Crown of Midnight, after a year of grueling labor in the Salt Mines of Endovier, Celaena has won the king's contest to become the new royal assassin. Yet the secret she keeps from everyone is that she isn’t at all loyal to the crown.

Maas first began working on Celaena’s story in high school, and she wrote whenever she could: “in math class, when I should have been doing homework, on vacations.” Though now a full-time writer, Maas says her actual writing process has stayed remarkably the same. “I need to have music on when writing; it helps me enter the scene, so I can visualize and feel it emotionally. Music keeps me going and inspires me.” Once Maas slips into that writing zone, she says “the story pours out of me. I shut out any doubts in my head and let the story come.”

Maas is also very linear in her approach. “I write from first word to last word. I don’t jump around. I’ve tried skipping ahead or jumping back. But I’ve found I need these carrot or cookie scenes that I am looking forward to writing—whether it’s action, a big reveal or a romance scene. Those are my rewards for writing the in-between. If I write those reward scenes first, I don’t feel motivated to write the in-between.”

Even when Maas is revising, she says she likes to feel her way forward, “going through the book as though I’m still a reader. I get very emotionally involved in my stories. If I’m jumping around, I lose that feeling.”

And most of all, as she has from the beginning, Maas trusts her assassin Celaena to help guide the story that she hopes will eventually unfold over a span of six or seven books. “I have an idea of where the series is eventually going,” she says. Maas has written about Celaena for so long that “when awful things happen to her, I feel awful for her. I know her so well at this point that I trust her and go in that direction….I know she’s in my head, not a real person, but at this point, I’ve learned to listen, even if it means changing the story.”

Managing the arc of these long and densely plotted adventures seems daunting. Maas is a fan of Moleskine notebooks, although she admits to retaining a lot of the forthcoming storylines in her head. Once again, music assists in her process. “My main way of organizing and keeping track of the material is through my playlists, which I make in order of events,” she explains. “As I’m daydreaming about the book, I’ll build the playlist, which is a scene-by-scene outline.” When material is moved around or a scene gets cut, Maas moves or removes that song from her playlist.

“I hate outlining traditionally! I didn’t consciously realize I was doing this until last year. I’d been outlining for years by making these detailed playlists,” she says. The playlists don’t just help her keep track of the plots she’s working on; they reflect the plot. “Now that I realize I do this, the playlists I make have become more important to me.” Maas estimates that the playlist she used to outline Throne of Glass includes about 270 songs; fans can find an abridged version of about 30 songs on her website.

Recently, Maas joined three of her close friends (also successful young writers) on a book tour with a twist. Maas says the Young Authors Give Back tour, which included free workshops for teen writers, was inspired by an impulse to share information about these authors’ writing journeys thus far. Maas and her good friend and fellow tour member Susan Dennard had enjoyed a memorable evening with legendary writer Robin Hobb, who’d taken them out to dinner at a book festival and shared stories from her own journey as a writer. “We were so touched by that—we were new authors and she was this fantasy writer of our generation,” Maas recalls. “She spent hours talking to us, answering our questions. We were nobody—just young authors, and she took us out to dinner and talked about how if you have success, it’s your obligation to pay it forward. So that’s how the focus of our tour shifted, not so much about promoting our books but more about paying this gift back through encouraging other young writers.”

If she could go back in time and give her teen self a bit of advice, what would she say? “Probably to be patient. That this journey takes a while—and that’s a good thing,” she responds. “It takes time to learn how to hone one’s writing ability. That would have been the hardest thing to believe at 16; I wanted everything right then, but having to wait and work for every little drop made it all sweeter, and it also made me a better person. Learning to face rejection and have to work for my dream gave me more strength than I realized, even at the time. That all really helped me evolve into who I am today.”

Maas stops and laughs. “But if I’d said that to my 16-year-old self, I would have said back, ‘Whatever.’ ”

Jessie Grearson is a writer and graduate of the Iowa Writers’ Workshop living in Falmouth, Maine.