It is not a documentary. It is not intended to be a documentary. It might have that "feel" but they are actors. It is even less of a documentary than what Michael Moore does.
Roger Ebert explained that was a brilliant technique because it indicated to the viewers that the producers could not find an evil enough person to portray McCarthy. Only evil could portray evil. I kid you not.
An interesting thing (at least to me) is that I saw it with my Dad who was a young adult at the time and who is very conservative still thinks McCarthy is bad and evil. When I tried to convince him that McCarthy had been proven correct, he thought I had too much giblet gravy at Thanksgiving. He said, "Everyone knows he was bad." The mainstream media will never admit the truth.