What impact did the house un-american activities committee (huac) hearings have on hollywood?

It changed the nature of free speech and free association. People could easily avoid being questioned if they would name someone who was a member of the communist party at any time. That turned friend and coworker against friend and coworker.

Eventually that immunity wore off because people were naming the same people. That's when things got really dicey. Now you were forced to either plead guilty or name someone else.

You can find out a lot more by searching out Dalton Trumbo. You can also search out Kirk Douglas' use of Tumbro's name for Spartacus. It was a very courageous move.