I haven't seen the Dalton Trumbo movie yet so I can't really comment on it at least in a credible sense and I'm not going to take the word of J.T. Hayworth and Allan Ryskind on it as well. Who both come from a very heavy right-wing partisan slant on it to say the least. Whatever you think of Dalton Trumbo's politics and I'm not a Socialist, democratic or otherwise, we're still talking about his politics here. The only reason why he was brought up in front of the House Un-American Activities Committee in 1947 was because of his politics.
The so-called Red Scare at the start of the Cold War that drove a lot of Americans crazy in believing that communism was so dangerous that a great liberal democracy like America, which is the opposite of communism, couldn't survive in the same world as communism. That it wasn't just communist policies and communist states that had to be defeated and destroyed, but communist beliefs as well. That you needed a fascist state coming from the Far-Right, where everyone looked at politics and the world from the same point of view and all shared the exact same values. That is not freedom and free speech, but a form of fascism.
Dalton Trumbo, wasn't brought in front of HUAC because he was a communist agent for the Soviet Union or a spy or something like the Rosenberg's. He was brought in front of HUAC because he was a Socialist in Hollywood and had a big mic and stage to get out his political beliefs. As much as the right-wing anti-Communist Warriors said they were defending freedom and liberty in the 1940s and 1950s, it would have been nice if only those things were true. Because instead they were defending what they claimed they were against. Which was fascism and statism.
You can say that Freedom of Speech is so important and that liberal democracy is so powerful that all views are welcomed to be expressed, because we're so strong as a country that we can tolerate extreme views from both the Far-Left and Far-Right and that Americans will be able to make up their own minds on these issues. But these things don't mean anything if you don't actually believe in them. What you do instead is say that freedom and free speech is so important that we have to stand up for the right for everyone to have their own views and be able to express them. Even if they're fringe and then hold them accountable for what they believe and say. But not try to shut them up and then we'll have true freedom and free speech.