HEY, ISN'T that the young Nelson Rockefeller commissioning Diego Rivera to paint a mural in the lobby of the Rockefeller Center?

And say, could that be Orson Welles directing a Federal Theatre Project troupe for an upcoming production called "The Cradle Will Rock"? And who's that fastidious little impresario behind Orson? Why, it's the young, very theatrical producer, John Houseman!

In "Cradle Will Rock," writer-director Tim Robbins's partially fictionalized fantasy, these and other leftists, rightists, capitalists and artists of the 1930s strut around New York City in quotation marks.

But none is particularly believable. Everybody's up to something political or famous, as if they were guests of "Meeting of the Minds," that TV show in which emcee Steve Allen interviewed actors playing famous figures from history.

While capitalists play footsie with budding totalitarians, Congress attempts to quell the influence of unions and communism on the American people. Uncle Sam's pet project becomes its favorite whipping boy. The Federal Theatre Project, a government program designed to bring low-cost theater to Americans, has become the domain of left-leaning artists.

Led by bilious Rep. Martin Dies (Harris Yulin), Congress probes the "un-American" tendencies of WPA theaters around the country, while its goon squad shuts down Welles's troupe -- sponsored by the federal program.

The problem with "Cradle" the movie isn't the casting. As a tell-it-like-it-is Rivera, Ruben Blades is rather entertaining. John Cusack makes a charmingly oozy Rockefeller. Newcomer Angus Macfadyen makes a wacky, eccentric Welles, while as his creative partner Houseman, Cary Elwes has a sort of campy-gay good time. And while we're on the subject, Susan Sarandon has her moments, too, as Italian propagandist Margherita Sarfatti, a party girl with a fascist mission: to drum up moneymen for Mussolini's war machine.

Despite Robbins's clear efforts to find ironies -- such as right-winger Sarfatti's friendship with leftie Rivera, or capitalistic Rockefeller's sponsorship of dangerously liberal art projects -- the movie is too diagrammatic to be any fun.

Its moral architecture becomes all too obvious when Welles's troupe defiantly performs "Cradle" (with the cast members singing from the audience seats), while Rockefeller dispatches his workers to sledgehammer that commissioned mural after he learns Rivera has included Lenin in the picture.

Another problem is a story that attempts to track a cast of thousands, including Emily Watson as a sort of grimy Little Match Girl who becomes involved with Welles's thespian federalistas; Vanessa Redgrave as the boho Countess La Grange with a yen for crazy causes; and Joan Cusack as the Dickensian-named Hazel Huffman, a miserable little shrew, who testifies before the house about commie-hugging, Negro-loving traitors in the theatrical field.

Well, thank goodness (as always) for Bill Murray. As Tommy Crickshaw, an alcoholic ventriloquist who falls in love with Huffman after helping her rehearse for that congressional performance, he steals the film from everyone. The great thing about Murray is the sense that, at any minute, he might just pause in the middle of a scene and say, "Oh, this is too ridiculous" and walk off the set. The existential ache in his eyes as he attempts to teach a pair of portly, bumbling adult twins the tricks of ventriloquism is priceless. "Your mouth . . . is moving," he says to one, unable to believe these idiots don't realize the most fundamental aspect of the art. The movie's real tragedy, for my money, wasn't the government's destruction of free speech, but Murray's limited screen time.

CRADLE WILL ROCK (R, 133 minutes) -- Contains nudity and obscenity. Opens Saturday at area theaters.