FOX News Channel's Bill O'Reilly (search) made a TV talk show appearance on what he said was "the worst day of my life" Thursday, vowing to fight sexual harassment charges by one of his producers.

Accuser Andrea Mackris (search) spoke publicly for the first time, saying she felt threatened by her former boss, who filed a lawsuit charging the woman and her lawyer with extortion.

Mackris, 33, said O'Reilly made a series of sexually explicit phone calls to her. Mackris, an associate producer on FOX News Channel's top-rated "The O'Reilly Factor," said he advised her to use a vibrator, told her about sexual fantasies involving her and engaged in unwanted phone sex.

During an appearance to promote his children's book on "Live with Regis and Kelly," O'Reilly said he'd been repeatedly threatened with lawsuits and bodily harm over the past few years. He said he knew that by filing his lawsuit, he could perhaps ruin his career.

"If I have to go down, I'm willing to do it," he said. "I'm going to take a stand. I'm a big mouth on the air and I'm a big mouth off the air."

On his own show Wednesday, O'Reilly called the case "the single most evil thing I have ever experienced, and I've seen a lot. But these people picked the wrong guy."

Mackris, who worked for O'Reilly for four years, said on ABC's "Good Morning America" that she took his actions and statements as a personal threat.

"I think my actions have been borne out in their actions toward me since I came out with it," she said.

ABC's Bob Woodruff (search) asked Mackris if she knew what she was up against. O'Reilly "is no shrinking violet," he said.

"Neither am I," she responded.

O'Reilly had come out with his lawsuit first on Wednesday, accusing Mackris and lawyer Benedict Morelli of seeking "hush money" during negotiations over the past few weeks. Mackris then immediately filed her harassment claim.

O'Reilly faces a tough legal hurdle with his own case, since the law generally protects employees from retaliation if they come forward with an abuse allegation, said Debra Katz, an attorney who specializes in sexual harassment cases for Bernabei & Katz in Washington.

"I think this is a crazy legal strategy," Katz said. "But I think they made the decision that the press strategy is more important here."

Another expert in employment law, Kenneth Taber of the New York firm Pillsbury, Winthrop, said O'Reilly would have to go a lot further to prove extortion than just saying Mackris made monetary demands.

But Taber also said Mackris' case could be undermined since she returned to work for O'Reilly after a brief stint at CNN — and after some of the alleged harassment took place.