When you ask writers how they came up with the idea for their first novel, some might say that it came to them in a flash. Or that they based the main character on someone they knew. Not Jenny Downham, the 43-year-old British author of Before I Die, a luminous story about a feisty 16-year-old girl who is dying of leukemia. She, in fact, heard voices.

But Downham isn't delusional. As a trained actress, she often gave the characters she played free reign over her body—including the ability to voice their opinions—so that she could completely inhabit each role. The more these characters talked to her, and the more she understood what it felt like to be someone else, the more authentic each performance became. Little did she know that this technique would translate so perfectly onto the page.

Downham started writing when she quit her job with a touring theatre troupe after the birth of her second son. Reluctant to take up a nine-to-five job and sacrifice more time with her children, she applied what she learned in the theater—character development, storytelling, conflict resolution—to writing.

In 2003, she submitted a chapter from the young adult book she'd been working on to the London Writers Competition. To her shock, she won first prize. By 2005, she had finished the book, and although it was never published, Downham was determined to keep writing and honing her skills.

That's when she “met” Tessa. What started out as a nagging voice inside Downham's head evolved into a full-blown character whose dire yet life-affirming story demanded to be told. Downham started keeping a daily journal for Tessa—a tool she had used as an actress to help her get into a role, which proved fruitful in writing as well.

“What would begin as my walk or my day in the café, would end up being Tessa's. I saw the world and the changing seasons through her eyes,” Downham recalls. Little by little, the pieces of Tessa's personality—her terminal illness, her list of 10 things to do before she died, and her determination to accomplish everything on that list—started to emerge.

After two years of living inside Tessa's head, Downham says she felt relieved—and a little saddened—when it was time to send her out into the world. Not to mention her fear that no one would be interested in hearing her story.

She needn't have worried. David Fickling at Random House fell in love with the book and crashed it onto his fall '07 list. Since publication, Before I Die has received glowing praise from the likes of the New York Times Book Review and Entertainment Weekly, garnered three starred reviews, and is a finalist for the Borders Original Voices program. Foreign rights have been sold to 19 countries and counting, and Blueprint Pictures has bought film rights.

When asked about her burgeoning success, Downham admits to feeling pleasantly surprised at how quickly the book took off—and how the applause doesn't seem to be dissipating. So what's next for this rising star? Although Downham didn't give away any major details, she did mention the presence of another voice murmuring inside her head—perhaps that of the main character in her next YA novel, currently under contract with David Fickling.