The controlling precedent in the United States was set in 1964 by the United States Supreme Court in New York Times Co. v. Sullivan. It is considered a key decision in supporting the First Amendment and freedom of the press.
A fairly high threshold of public activity is necessary to elevate a person to public figure status. Typically, they must either be:
A person can become an "involuntary public figure" as the result of unwanted publicity. For example, a person accused of a high profile crime may be unable to pursue actions for defamation even after their innocence is established on this basis.
A person can also become a "limited public figure" by engaging in actions which generate publicity within a narrow area of interest. For example, jokes about Terry Rakolta, an activist who spearheaded a boycott of the show Married With Children, are fair comments within the confines of her public conduct and are protected because she was a "limited public figure."