An ardent agnostic, screenwriter Dalton Trumbo was not a Ten Commandments kind of guy. He was devout about the Ten Amendments to the U.S. Constitution, most especially the first in that Bill of Rights, with its guarantee of free speech.

Trumbo, a rousing documentary as ornery, orotund and captivating as its subject (1905-1976), is an anatomy of irony. This cracking account chronicles how invoking the First Amendment got the puckish screenwriter convicted of contempt of Congress in 1950.

As he was the first to admit, the celebrated writer of Roman Holiday and Spartacus was guilty of contempt. But it was Congress that first had trampled on his First Amendment rights by insisting that Trumbo, a member of the Communist Party during World War II, when the Soviet Union was a U.S. ally, identify others who were now or had ever been members of the Communist Party.

Peter Askin's engaging chronicle, adapted from an Off-Broadway play by its subject's son, Christopher, is a Trumbonanza. Instead of dramatizing the events in his life, actors including Michael Douglas, Paul Giamatti, Liam Neeson and Nathan Lane (who played Trumbo on stage) read from the screenwriter's letters, drenched both in the political ferment of the 1950s and the bouquet of the writer's thoughts.

While this may sound like a movie made for radio, the effect is unusually involving. What better way to get inside a writer's head than through his own words? Complementing this are clips from Trumbo-written films that resonate with democratic and idealistic fervor.

Askin provides a thumbnail history of Trumbo's life and career highs. Prior to the 1947 congressional tribunal, he wrote novels, including Johnny Got His Gun, and many Oscar-nominated screenplays, including the superpatriotic A Guy Named Joe and 30 Seconds Over Tokyo. He includes the still-shocking film clips of the House Un-American Activities Committee (HUAC) where lawmakers scoff at Trumbo when he argues that his political beliefs - and defense of them - are protected by the First Amendment.

Many of the letters used were written during his 10-month prison stint for contempt of Congress. There's a poem for Christopher's birthday, a tender love letter to his beloved wife reminding her that if as a single mom she is frustrated with the kids, she should console herself with the memory of their conception.

Because Trumbo didn't cooperate with HUAC, he was blacklisted - made unemployable by Hollywood studios - and forfeited a decade of career prime time. He wrote scripts, including Roman Holiday, which won an Oscar, soliciting friends who passed them off as their own. In 1956 his bullring screenplay, The Brave One - written under the alias of Robert Rich - won an Oscar that he could not claim. Finally in 1960, Kirk Douglas, executive producer and star of Spartacus, broke the blacklist by giving Trumbo screenwriting credit.

Like Trumbo's career, the documentary is tragic, comic and incisive, and a reminder to Americans of the importance of fighting for the founding principles of the nation.

Trumbo *** (out of four stars)

Directed by Peter Askin, written by Christopher Trumbo, based on his play. With Joan Allen, Kirk Douglas, Michael Douglas, Paul Giamatti and Liam Neeson. Distributed by Samuel Goldwyn Films.

Running time: 1 hour, 36 mins.

Parent's guide: PG-13 (playful sexual references)

Playing at: Ritz at the BourseEndText

Contact movie critic Carrie Rickey at 215-854-5402 or crickey@phillynews.com. Read her blog, Flickgrrl, at http://www.philly.com/philly/blogs/