This act made it unlawful to plan any action that might lead to the establishment of a totalitarian dictatorship in the US. Truman originally vetoed, but Congress enacted the law. http://www.encyclopedia.com/topic/McCarran_Act.aspx
The HUAC was interviewing the film industry. The HUAC interviewed 41 people and voluntarily answered the questions. There were ten people who refused to be interviewed and claimed the hearings were unconstitutional. They were then sent to prison for their refusal. http://www.spartacus.schoolnet.co.uk/USAhollywood10.htm