George Hamilton later complained that "Schary de-ethnicized the entire production and took out the brilliance for good measure".
In 1929 young Brooklynite Moss Hart, influenced by the great playwrights, devotes his leisure time to writing for the theater. Failing in his aspirations, however, he accepts a job as social director in the Catskills and then stages plays at the YMHA in Newark.
Eventually he takes the advice of agent Richard Maxwell and writes a comedy, Once in a Lifetime, which deals with the early days of Hollywood films, despite the fact that his knowledge of the movie industry is derived from the pages of Variety.
After being subsidized by a friend, Joe Hyman, he sends the manuscript to producer Warren Stone, who promises a decision within a week. When months pass without any word, Hart's friends sneak a copy of the play to Sam Harris, who agrees to produce it if George Kaufman will collaborate on the script and also direct.
Although Kaufman consents, the Atlantic City opening is a failure, and he considers quitting until Hart comes up with an idea that both men feel will turn the play into a hit. It finally opens to rave reviews in New York City in September 1930, thus beginning the long-lasting Kaufman-Hart collaboration.
Film rights were bought by Warner Bros who assigned George Axelrod to write the script. Eventually the project went to Dore Schary, who had known Hart for a number of years.
"I've tried to deal with Moss as I knew him," said Schary. "The film is more about character than the theatrical world. But I think his story represents more than just a guy trying to success in a tough, creative field, It's about his frustrations in trying to reach a dream, and then it isn't what he expected when he gets there. You might call it a typical American theme."
Dore Schary himself was the basis for the character "David Starr" (played by Sam Groom).
Anthony Perkins and Dean Jones were early contenders for role of Moss Hart.
George Hamilton was socially friendly with the family of Moss Hart and Hart reportedly wanted Hamilton for the role.
To promote the upcoming release of Act One, George Hamilton appeared on a September 1963 episode of I've Got a Secret, a prime-time game show in which a panel of celebrities attempted to discover the guest's "secret." Hamilton's secret? The actor identified as Hamilton and grilled by the panel (who failed to guess his secret) was that he was not actually Hamilton at all but instead a dark-haired handsome sort-of-look-alike pretending to be Hamilton. The real Hamilton showed up at the end of the spot and earned the admiration of panelist Henry Morgan who expressed astonishment that any performer of Hamilton's stature was secure enough to take part in a stunt which, in essence, pointed up the fact that he was unrecognizable to a quartet of supposedly in-the-know celebrities.
Kitty Carlisle, Hart's widow, was unhappy with the film. When Schary showed it to her she said diplomatically, "Well, you did it," and later said she "we draw a veil" over the film.
James Woolcott wrote about the movie in Vanity Fair:
Act One has a very 50s feel, more of a boxy affinity with the Golden Age of TV than anything released in a film canister. It abbreviates the birth pangs and floor-pacing agonies of Once in a Lifetime’s gestation, the torturous rounds of re-writes and previews, sugarcoating everything about the romance of the theater that All About Eve had salted and pickled... Hamilton isn’t that bad, but playing an underdog of raging literal and metaphorical appetite, he purrs as a screen presence, his matinee-idol profile belying his character’s self-doubt. Nothing needy thumps around inside him. (Had Act One been made a decade later, Richard Dreyfuss would have been perfect.) What makes this Act One work are the crafty scene-stealers cast against Hamilton’s ingenuous Hart: Eli Wallach... Jack Klugman... and, most of all, Jason Robards as George S. Kaufman. With high-top hair, skeptical eyebrows that lift like Groucho Marx’s, and a resigned posture suggesting a body that’s a dried rind, Robards’s Kaufman is an Al Hirschfeld caricature come to life. Wallach, Klugman, and Robards—each had a distinctive grain to his voice, a variable velocity in his delivery. The contrast between these shrewd operators and the freshman crew playing Hart’s smart-aleck pals—among them future star George Segal as Hart’s personal prophet of doom—gives the movie its rustling texture as a Hollywood artifact, nearly everybody in it destined for greater glories on-screen.