J.K. Rowling on 'The Casual Vacancy,' Using a Disguise, What She Learned from Michael Jackson

Sept. 27, 2012— -- J.K. Rowling has found success few authors could dream of, but managing her worldwide fame is something the notoriously private author still wrestles with as she prepares for the release of her highly-anticipated adult novel, "The Casual Vacancy."

"I once have used a disguise," Rowling, 47, said in an exclusive and rare television interview with "Nightline" anchor Cynthia McFadden in Edinburgh, Scotland. "It was effective. It's how I managed to buy my own wedding dress. But I am not going to tell you what it was, in case I have to use it again. But it did work."

Rowling's Harry Potter books have sold more than 450 million copies, been translated into 73 languages and produced eight blockbuster movies, making her the first billionaire author. But the woman who built an empire using made-up words such as, "codswallop," "Hufflepuff" and "veritaserum" left that magical world behind to write about a much grittier place, one called Pagford, where characters use words like "condom," "heroin" and "vagina."

The frenzy of interest in "The Causal Vacancy" has the publisher guarding it like the Sorcerer's Stone, keeping its plot under lock and key until its Sept. 27 release date. (McFadden and ABC News producers read the manuscript, kept protected in the publisher Little, Brown and Company's New York office).

Rowling's world hasn't been ordinary for quite some time. She was on a train when she first dreamed up her idea for a story about the boy wizard in 1990. "The Casual Vacancy" came to her five years ago while she was on a private jet, touring the United States to promote the last Harry Potter book.

"When I first became, what I called rich, and actually, that was way before a lot of people would have called me rich, it was uncomfortable," she said. "The shift was so dramatic. I found it very disorientating. I felt guilty, strange, out of order, didn't know what to do, was scared I was going to blow it in some way and my daughter's security would be gone."

It's a fear that became very real when Rowling discovered that a journalist had managed to slip a note into her 5-year-old daughter's school bag one day.

"That was pretty much the worst thing that ever happened to me," Rowling said. "It felt like something the secret police would do, just to prove they could get there.... To this day, I think that was the worst thing."

Last year, Rowling, along with actors Hugh Grant and Sienna Miller, testified before a judge-led committee investigating Britain's aggressive tabloids. Rowling said she grappled with the decision to testify for some time.

"I thought long and hard before doing that," she said. "Because you're in this paradoxical situation where you're sitting up in front of live cameras, and in the interest of your privacy, you're kind of invading your privacy. But…it was quite healing."

She even considered writing "The Casual Vacancy" under a pseudonym, but decided against it in the end.