The long career of the great musical theater composer Richard Rodgers was split neatly into two phases. In the 1920s and ’30s, he wrote a string of hit Broadway shows with the lyricist Lorenz Hart, and from the early 1940s through the late ’50s, he did much the same with lyricist Oscar Hammerstein II. The former partnership gave the world some of the pillars of the so-called Great American Songbook (“My Funny Valentine,” “The Lady Is a Tramp,” “Where or When”), though the frothy shows they were written for are seldom revived. The latter duo became a brand name, better known for creating canonical musicals, from “Carousel” to “The Sound of Music,” than for individual songs.

The new movie “Blue Moon,” named for one of Rodgers & Hart’s most enduring tunes, is set at the pivot between these two eras—a change that is about more than theatrical fashion but also reflects a larger cultural shift, from Jazz Age sass and Depression-era gloom to post-World War II patriotism and conformity. At the film’s center is not Rodgers himself but Lorenz Hart, cast here in the role of jilted partner.

It is the opening night of “Oklahoma!” in 1943, the show that will inaugurate the Rodgers & Hammerstein juggernaut and effectively leave Hart behind. (He would collaborate on a few more songs with Rodgers before dying later that year of pneumonia at the age of 48.) We find Hart (Ethan Hawke) licking his wounds at the theater district watering hole, Sardi’s. As he waits for the “Oklahoma!” opening night party to arrive, he confides in an understanding bartender (Bobby Cannavale) and commiserates with a quiet fellow wordsmith, E. B. White (Patrick Kennedy). On his mind is not only the impending end of his partnership with Rodgers but his romantic hopes with a 20-year-old student, Elizabeth Weiland (Margaret Qualley), with whom he’s been exchanging ardent letters.

What ensues, under director Richard Linklater’s smooth guidance, is a witty, involving, if somewhat lopsided single-set drama—essentially a filmed play. Written by Robert Kaplow, with whom Linklater made the underrated backstage gem “Me and Orson Welles,” “Blue Moon” is dense with argument and insider dish (and showbiz Easter eggs). It is on its surest footing when portraying the fragile, often volatile nature of creative collaboration. The scenes between Hart and Rodgers (Andrew Scott) bristle not only with aesthetic disagreement—Rodgers is sincere about the new, more earnest direction of his work with Hammerstein, which Hart views as corny Americana—but with the well-worn ruts of a foundering artistic marriage in which the partners know each other all too well. Hart, an infamous alcoholic prone to destructive binges, pleads with the punctilious Rodgers for another chance, but his former partner has clearly moved on.

Less successful is the film’s focus on Hart’s apparent lust for a nubile young woman, which for much of the film’s running time seems borderline creepy. This turns out to be an elaborate misdirection, as Hart was a closeted gay man who also had intermittent, non-sexual infatuations with women. And its payoff is a riveting scene opposite a distraught Qualley, in which Hart’s romantic longings are exposed as a kind of wishful sublimation, and for once this irrepressible talker is a rapt, heartbroken listener. As one of his lyrics put it, “You are too beautiful/ And I am a fool for beauty.”

At its best, “Blue Moon” works like this, moving fleetly from barroom banter to wrenching heart-on-sleeve emotion, in much the same way a great Rodgers & Hart song can sweep you efficiently from bubbly diversion to deep feeling. I think of one of my favorite Hart lyrics, from “I Didn’t Know What Time It Was,” in which the terse cleverness of the rhymes somehow makes the song’s sudden burst of sincerity pop all the more:

I didn’t know what year it was

Life was no prize

I wanted love and here it was

Shining out of your eyes

To seal the deal, Rodgers’s melody leaps up on the last word. It’s a case study in how a song can move you.

But a film is not a song, and “Blue Moon” finally stands or falls on its central performance. How does Hawke fare? Playing flagrantly against type, he has thinning black hair desperately combed over a balding pate, and his scenes have been shot and staged to approximate Hart’s diminutive stature—what Rodgers once called his partner’s “gnome-like appearance.” It’s a role, honestly, that an actor like Stanley Tucci could walk away with. Viewed generously, though, this is a case in which the clear effortfulness of Hawke’s performance subtly rhymes with the strenuous labor Hart seemingly put into maintaining his bon vivant facade.

Indeed, “Blue Moon” tells the familiar story of an entertainer whose relentless repartee is meant to distract us from his misery, and, if he’s really good, our own. When said entertainer also happens to be a great artist, as Hart was, that humanity can’t help but peek through. “Blue Moon” captures both the buzzy chatter that Hart wove to stay afloat and the melancholy that ultimately kept him earthbound. “I fell in love with love, with love everlasting,” as one Hart lyric put it. “But love fell out with me.”

Rob Weinert-Kendt, an arts journalist and editor of American Theatre magazine, has written for The New York Times and Time Out New York.