At the Four Seasons Hotel in Beverly Hills recently, author Janet Evanovich joined Katherine Heigl for a joint interview with USA Today about the movie adaptation of her novel One For The Money.

For years, there had been a mystery the novelist couldn’t solve – finding the perfect Hollywood actress to embody crime-busting protagonist Stephanie Plum, the heroine of her best-selling series of books. That changed instantly in 2008 when Janet watched Katherine in 27 Dresses. After seeing her strait-laced character rush from a bar cursing at the top of her lungs, "I just knew it," Evanovich says to Heigl. "From that instant you were Stephanie Plum to me."

After a long wait for a film adaptation to arrive (Evanovich sold the rights to One For The Money in 1994), fans of the book series were surprised to hear that Katherine – famed for her blond hair – was set to play Plum who has always been strictly a brunette. "They had a hard time with it because (Heigl’s) blond," Evanovich explains. "But 90% of them were just so excited that the movie was going to get made."

Katherine tried dyeing her hair, but then had to revert to plan B to get the desired look. "I wigged it," she explained. "Sorry."

"I wouldn’t have cared. Stephanie is not her hair," the author explained to USA Today, before deadpanning to Heigl: "But my fans would have run you out of town."

Hence the lengths Heigl went to for the brunette ‘do in One for the Money. She went from dyeing her hair, months before she was due on set, to wearing a wig. (Says Heigl: "There were all these online debates, do I look better as a blonde or a brunette? I ignored it.")

The wig itself caused its own little drama. "The producers were really nervous about it," Heigl says. "It was a big deal."

Katherine pulled hair inspiration from the likes of Julia Roberts and Taylor Swift for the stylish creation, which arrived on set the day of a major Money screen test. When she rocked the new wig for the producers, the relief was palpable.

"I think they thought I was going to come out looking like Bozo the clown," says Heigl. "You could tell there was so much relief that I wasn’t doing something completely bananas."

Producer Gary Lucchesi calls the resulting look "perfect." Heigl says it’s "my Julia-Roberts-does-Trenton look." Now that she’s back to her usual blond hair, she misses her foray into brunette-dom.

"I feel more sophisticated as a brunette, more European," says Heigl. "And with that brunette, I felt more Jersey."

Katherine Heigl (Stephanie Plum) & Jason O'Mara (Joe Morelli) In One For The Money Katherine Heigl (Stephanie Plum) & Jason O’Mara (Joe Morelli) In One For The Money

Hair done, Katherine moved onto the finer aspects of bounty-hunter training. She took her first trip to a gun range where she proved to be a sure shot. ( "I called my dad and kept my target to show him," she beamed). Lucchesi says Heigl also showed impressive skill behind the wheel of a three-ton, stick-shift truck in another scene.

Still, there were some low points while delving into the New Jersey crime business, such as Handcuffing 101. "Ugh," says Heigl. "I could never get that smooth hand-cuffing motion."

Discussing her characters Jersey accent, Katherine amitted "It was nerve-racking. 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 also played into her dedication to keeping the movie true to the book.

"I became very possessive of Janet’s material," says Heigl. "I was really loud about how important it was to honor the book."

Evanovich, meanwhile, stayed in the dark during the film process. "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 once she finally saw the completed film. Evanovich concedes that she too fretted about the final result and put off the screening for months.

"I was terrified to see it," says Evanovich. "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."

The author’s stamp of approval caused the filmmakers to literally pop the Champagne.

"That was the best call we got in the entire process – that Janet loved the movie," says Heigl, exhaling dramatically. "We were like, ‘Thank God!’"

Katherine Heigl & Janet Evanovich Katherine Heigl & Janet Evanovich (Photo: Larsen & Talbert)

Movie wrapped, Heigl reached out to Evanovich and the two immediately talked over the phone for an hour and a half for the first time about everything from dog rescues to kids. This led to an instant e-mail relationship.

"I get the funniest, quippiest e-mails from Janet, none of them appropriate for sharing," says Heigl. "I don’t have to censor myself. I can just be me."

"That’s another thing we have in common," adds Evanovich. "I’m from New Jersey. I have an extensive vocabulary. I’m totally politically incorrect."

After their first in-person meeting for this interview, the two fast friends even escaped back to Katherine’s Los Feliz, CA, house for a home-cooked meal that lasted late into the evening. Evanovich reported back that she was especially tickled when the Hollywood star searched in vain to find all the ingredients for a Martini.

"It was like Stephanie searching through her apartment for a Snickers bar," says Evanovich. "I knew she was one of us."

"She had the vodka, she had the chilled glass, but she couldn’t find the vermouth," the author adds. "It showed the human side of a person who has a lot going on in her life." The ingredients for a suitable Martini were eventually found ("She located an olive and we were good to go," says Evanovich).

The read the interview in full please visit USA Today and look out for more in today’s edition of the newspaper.