LOS ANGELES -- For years, there was one mystery author Janet Evanovich couldn't crack: finding the right Hollywood actress to embody Stephanie Plum, the crime-solving heroine of her best-selling books.
A major break occurred when Evanovich saw 2008's" 27 Dresses," notably the scene where Katherine Heigl's straitlaced character rushes from a bar cursing at the top of her lungs. Right then, everything clicked, Evanovich says.
Today, Heigl, 33, will be Stephanie Plum to the rest of the world when the film adaptation of Evanovich's first novel, "One for the Money," hits theaters. Seventeen years after the book was released, the actress finally will put a face to the lingerie saleswoman-turned-bounty hunter who has sparked a behemoth 80 million copies in book sales (including nine No. 1 USA Today best sellers) and an audience of diehard fans.
"I don't know why it took so long," says Evanovich, 68, whose latest and 18th Plum novel is "Explosive Eighteen" (Bantam, $28). "Every time I write Stephanie Plum, it's going to be Katherine Heigl's face there. She totally nailed it."
Amazingly, Heigl's casting had nothing to do with Evanovich's "27 Dresses" epiphany, because the author had relinquished any say in production matters when she sold the rights to "One for the Money" before it was published in 1994. Despite the early sale, there was no concrete movement on a film adaptation for years. Evanovich continued churning out new, increasingly successful novels, while actresses' names occasionally surfaced as potential players.
"Every now and then, I would get a phone call that would be: Jennifer Lopez is going to be attached to this," Evanovich says. "Or Reese Witherspoon. There was never anybody really there." Her fans clamored for Sandra Bullock. "They could see her with the dark hair. And she has great comic timing."
The author, who was a successful romance writer before moving to mystery writing in the early '90s, never had a Hollywood personality in mind for the sassy character she was creating. Stephanie featured a little bit of Evanovich herself and some of her daughter Alex, then 20.
On the set of 2009's "The Ugly Truth," producer Gary Lucchesi had presented Heigl with a copy of "One for the Money." The avid reader was hooked.
"Two months later, someone was able to get me to come out of the bedroom after I had gone through 10 books," Heigl says. "(They are) so addicting."
Lucchesi and Lakeshore Entertainment acquired the rights, with Heigl to star as Plum. But a major hurdle lay ahead: Fans were not pleased that Heigl is famous for her blond hair, while Plum is a brunette.
Heigl tried dyeing her hair, but had to go to a plan B to get the desired look. "I wigged it," she says. "Sorry."
Even tougher was her first on-camera accent -- and a broad one, to boot.
"It was nerve-racking," she says. "There were moments I saw it wavering. I'm normally the jerk in the audience who goes, 'Blah, they totally lost the accent there.' This was karmic."
Her attention to detail played into her dedication to keeping the movie true to the book. "I became very possessive of Janet's material," Heigl says. "I was really loud about how important it was to honor the book."
Evanovich, meanwhile, stayed in the dark during the filming. "I always felt, once it goes into movie land, the book belongs to someone else," she says.
So neither producers nor Heigl had any idea what Evanovich would think when she saw the completed film. Evanovich concedes that she fretted about the final result and put off the screening for months.
"I was terrified to see it," she says. "But when I did, it was everything I could have wanted and more. I was almost in tears when the movie ended. I was so relieved."
With 18 novels in the can -- and counting -- the franchise could lead to unlimited possibilities.
Or a new set of problems, since Plum remains 32 years old throughout the series -- a tough feat to mimic, even for Heigl.
"This could be a dilemma. I'll have to keep my face frozen with Botox so I can be ageless for years," Heigl jokes. "Jason (O'Mara) and I were talking about it. It's like, 'Oh my God, if we do all 18 of these books, how old will we be⢠How will we pull this off?"'
"But if we shoot six at a time," she says in a light-bulb moment. "There we go."

