There's a delicious moment in Trumbo, the new biopic about the blacklisted screenwriter, when Dalton Trumbo (Bryan Cranston), serving a year in a correctional facility in Kentucky for contempt of Congress, crosses paths with J. Parnell Thomas, the former chairman of the House Un-American Activities Committee (HUAC).This is 1950 and the portly Thomas, a Republican Congressman from New Jersey, has been convicted of fraud after his secretary sent incriminating documents to a newspaper (I can just hear her: "make your own damned cuppa coffee!"). Parnell (James DuMont) is mopping the prison floor when Trumbo walks by. Both pause, taking in the moment. "Well, look at us, just a coupla jailbirds," says Thomas, with a hint of a smile. "Except you actually committed a crime," says Trumbo.

It's such a good scene, but it never happened. They were in different prisons. Thomas was in Connecticut, along with two of his other victims - Ring Lardner jnr and Lester Cole. History is never as convenient as screenwriters need it to be, so they make stuff up. It gets them into trouble but in the case of Dalton Trumbo, the most successful and highest paid scriptwriter of his era, the truth caused him infinitely more trouble.

Trumbo joined the Communist Party of the USA in 1943, at a time when the US and the USSR were allies against Hitler. Six months after he joined, the CPUSA boasted 80,000 members – an all-time high. All of those in what became known as the Hollywood Ten – eight writers, two directors – were also current or past members."People joined the Communist Party because it was doing something,' Trumbo said in an interview in the 1960s. "It was an effective instrument. It was doing things that they thought should be done. It was opposing the rise of fascism all through Europe, it was helping those who were refugees…"

At home it opposed segregation, supported the war effort and ran successful candidates in city elections. Throughout the HUAC hearings, the subsequent jailing of the Hollywood Ten and the witch-hunts that followed, the CPUSA was always legal. It just wasn't popular in a country where foreign "isms" were never popular (as opposed to Isolationism and Americanism, which were).

Republicans took control of the Congress in 1946, which allowed the corrupt and aggressive Parnell Thomas to reinvigorate the 10-year-old HUAC committee. He picked the perfect target: reds in Hollywood. The idea that communist writers were planning to take over Hollywood, or filling movies with subversive messages, was as ludicrous then as it is now, but attacking Hollywood gave maximum profile. HUAC was able to roll together anti-Semitism, anti-communism and anti-unionism, a winning combination for the media. Hedda Hopper (played in the movie by Helen Mirren) became judge and jury in her Hollywood gossip column: none of her 35 million readers liked a rich, smart-arse commie like Dalton Trumbo.

The new film is a treat – witty and personal, led by a masterful performance from Bryan Cranston, with Diane Lane as his wife Cleo. Trumbo often composed his scripts in the bath, whiskey in one hand, cigarette in the other. Before HUAC, he was making $75,000 per script. After the studios caved in and sacked everybody who had been named as a communist, he spearheaded the covert fightback, organising jobs for blacklisted writers to write under false names for poverty-row studios. Trumbo won two Oscars (for Roman Holiday and The Brave One) using fake identities.

What then is the legacy of HUAC? Did the witch-hunts change Hollywood? Trumbo was eventually vindicated and honoured before his death in 1976 (from lung cancer – no surprise). Everyone moved on, didn't they?

Not quite. HUAC was a wrecking ball that changed Hollywood forever, by showing what would happen to anyone who got out of line. Many of the talented progressives scattered around the globe, ducking for cover (like Joe Losey and John Berry, who went to Europe). Those who stayed were cowed into subjection, like Edward G. Robinson and Edward Dmytryk, who eventually named names. HUAC enshrined a fearful conservatism in American cinema of the 1950s, a sort of political lobotomy. No surprise that the Biblical epic rose in this decade: God was the only safe bet.

HUAC may have hastened the demise of the studios, even after they kowtowed. When the courts ruled in 1948 that studios could no longer own theatres, the edifice began to crack. A number of big names – Bette Davis, Frank Capra – set up as independents, a direct challenge to studio power, initiating the model that pertains today. There was no love lost between these two individuals and the studios before HUAC, but the blacklist destroyed any good faith that was left among others. HUAC didn't just shatter the lives and careers of the radicals; it destroyed many of those who testified, like the actor Larry Parks, who became a pariah. The enmities lasted for decades.

Fear eats the soul, as Rainer Werner Fassbinder said. HUAC showed Hollywood that Washington could play rougher than they could. The First Amendment to the American Constitution (the right to freedom of speech and religion) counted for nothing against Thomas and his committee. Thomas went to jail for diddling the tax system, not for raping the Constitution. Stalin must have been amused: HUAC stole his idea for show trials. HUAC was more Stalinist than the CPUSA could ever be.

Twitter: @ptbyrnes