Film director Zhang Yimou says the script of The Flowers of War is the best he's encountered.

Based on Yan Geling's 2005 novella of the same title, the film tells the story of an American priest, a group of Chinese schoolgirls, prostitutes and soldiers finding refuge in a church and risking their lives as they struggle to survive the Nanjing Massacre in 1937.

Yan says the core of the story germinates from the diaries of Minnie Vautrin, an American missionary renowned for saving the lives of many women at the Ginling Girls College during the Nanjing Massacre.

Yan has often taken part in events commemorating the Nanjing Massacre since 1993. "Each time I returned with an impulse to pen a story about it," she says.

It was at those events that she met Iris Chang, the American historian and journalist best known for her 1997 The Rape of Nanking.

Yan says Chang's account of the Nanjing Massacre was a "great help" to her, as it gave her a profound understanding of Japan.

In 2007, one of her friends, who was working with Zhang Yimou, recommended the story to the director. Zhang was interested and decided to adapt it into a film.

Zhang said in a previous interview that the story attracted him because of its unique perspective - delivering the theme of redemption through 13 prostitutes.

Yan and scriptwriter Liu Heng worked on the script, producing several drafts, following Zhang's suggestions.

Yan says working with Zhang was a happy experience from beginning to end. "He is easygoing and energetic," she says. "He likes telling stories, and some of them are really inspiring."

She took advantage of the historical materials collected when revising the script, and turned the 40,000-word novella into a 120,000-word novel earlier this year.

"I want to convey the idea that only after extreme events do men begin to know their real selves and complete their growth and transformation," she says.