Whether they are public figures or not, you are legally permitted to photograph anyone you want, and you are free to write about anyone you want. This includes union oficials and shop stewards.
Of course, this does not mean that there are no restrictions in how you photograph or write about someone. You may not break into someone's house to take a picture of them, and you may not make defamatory statements about them.
In the event you do cross some sort of line in your documentary pursuits, you may have increased legal protections if the subject is a "public figure" in a legal sense, in which case the subject would need to prove actual malice to obtain a defamation judgment.
There are several types of public figures:
public officials, which includes probably all elected officials, as well as those who occupy government positions with "such apparent importance that the public has an independent interest in the qualifications and performance of the person who holds it, beyond the general public interest in the qualifications and performance of all government employees," Rosenblatt v. Baer, 383 U.S. 75 (1966);
all-purpose public figures, people for whom there is "clear evidence of general fame or notoriety in the community, and pervasive involvement in the affairs of society," legal remedies for defamation become less necessary, Gertz v. Robert Welch Inc., 418 U.S. 323 (1974); and
limited-purpose public figures, who are treated as public figures in cases where "the nature and extent of an individual's participation in the particular controversy giving rise to the defamation" indicates that such treatment is appropriate, id.
So there are many different kinds of public figures. One person could be a private figure in one case and a public figure in another, or he could be a private figure one day and become a public figure the next. Donald Trump, for instance, was a private figure as a child, but at some point early in his career, he became at least a limited-purpose public figure, where he would have a harder time bringing a libel case based on allegations about his business conduct.
Eventually, through the growth in his business and media empire, as well as his deliberate pursuit of media coverage, and no later than the time he became a candidate for president, he became an all-purpose public figure, requirnig him to prove actual malice in any case.
Finally, upon inauguration to the presidency, he became a public official, which carries that same increased evidentiary burden in defamation cases.