Mary Higgins Clark, the bestselling author who has written dozens of page turners, has two rules when beginning to write a suspense thriller.

"You have to know who did it ... and why he or she did it," the author told CNBC's "On the Money" in an interview. Yet while she always knows the culprit, "I don't know the final scene when I start the first. I have to figure that one out."

In a prolific writing career, Higgins Clark has penned more than 50 books, selling more than 100 million copies in the U.S. alone. As for the actual writing, she says she gets help from the characters she creates, and the people in her stories.

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"Once I know the people, they tell me where to go now and what's the next," Higgins Clark said. "I mean, I can be writing and a character will say, 'I don't belong in this scene, I'm getting out.' And that's when writing is fun."

Yet the path of the "queen of suspense" wasn't direct. She had a couple of stops early on in 1949 and 1950, when she worked as a Pan American World Airways flight hostess.

"That was extremely glamorous in those days," Higgins Clark told CNBC, "I mean, flying was still so new." She worked Pan Am routes to Europe, Africa and Asia, and recounts that the experience was rare, "for 21 years old, to be all over the world in those days was pretty nifty."

Before that, the Bronx, New York-born author had a stint as a telephone operator at a Manhattan hotel. "Hotel Shelton, Good Afternoon," she described how she would operate the switchboard.

"And then I'd listen in" on phone calls, Higgins Clark said with a laugh. There was the "lady of the house" at the hotel and she'd always listen in on her, "because I loved to hear her make her dates."

Fast forward to her early 30s, and Higgins Clark was married with a family. She was writing then, but having a hard time getting published. "There was no market for short stories," she said.

After her husband died, however, the need to turn writing into a livelihood suddenly became more urgent. "I was a young widow. I had five kids to support," she added. "I said, 'I've got to make money.'"

Higgins Clark remembered a defining moment clearly. "I looked at my bookshelf and I realized I loved suspense more than any other subject. I'd always been reading thrillers and knew why the good authors work and their stories work and why others don't work."

At that point, she tried her hand at suspense writing. Her first, "Where Are the Children?," was published in 1975. "It became a best-seller and launched me, thank heaven."

Forty years later, her 51st book, "All Dressed In White," co-written with Alafair Burke, is out now. She acknowledges her writing style and "brand" have some factors in common.

"In my case, it's always a woman, a young woman," she said.

"Smart, intelligent and something happens. She's not on the wrong side of town at four in the morning. She's living her life and something crosses it," Higgins Clark added. "And by her own intelligence, she works her way out of it."

She called it a "basic theme ... with different situations, of course. Because you never want to repeat yourself."

Over the decades, she's seen the publishing business evolve. Mainly, she says, "people aren't reading as much. It's very simple." She cites the Internet, Facebook and Twitter, among "so many diversions" that have "changed publishing. But people still read, of course, thank God."

While currently at work writing her 52nd book, "As Time Goes By," the author sees more books in her future.

"Suppose I just said, 'I'm retired.' And I would go, 'now what do I do,'" she asked. "Because I love to write."

"On the Money" airs on CNBC Saturdays at 5:30 am ET, or check listings for air times in local markets.