By the time Kate Winslet, as Mildred Pierce, sat down in front of the radio at the end of the fourth episode of HBO’s recent miniseries, the slow-moving, rather leaden series had already established itself as Wagnerian in its pace. At this point in the action, opera made an explicit entrance: Mildred’s daughter, Veda, was singing on a radio show, having, in a feat requiring considerable suspension of disbelief, reinvented herself as a coloratura soprano.

Obviously, this setup requires that the radio performance be really good. “Mildred Pierce” pulled it off: The music — the “Bell Song” from Delibes’s opera “Lakme” — was so well sung that not only was Mildred transfixed, but viewers were likely to prick up their ears as well. Who was that singer? (Answer: the former star Korean soprano Sumi Jo, in a recording several years old.)

Opera in film is always a signifier: You don’t go to the trouble and expense of dragging such an artificial art form into your opus without a pretty good reason. Generally, film directors use opera in one of two ways. Opera (and orchestral concerts) sometimes appears at the climactic moments of a thriller, with the onstage action illustrating or paralleling the offstage suspense (as in “The Man Who Knew Too Much,” “The Godfather Part III” or the 2008 James Bond movie “Quantum of Solace,” which set Daniel Craig backstage at a Bregenz Festival’s “Tosca”).

Opera’s other main film function is to symbolize high class, glamour, emotions beyond the compass of mere mortals: all the things that you, in your humdrum life, are not.

Julia Roberts, in “Pretty Woman,” shows that she has the ability to appreciate the finer things in life by crying at the opera. Cher and Nicolas Cage, in “Moonstruck,” have opera as a reflection of and trigger for their own over-the-top love story. And Mildred, in “Mildred Pierce” — both in Todd Haynes’s miniseries and in the original book by James M. Cain — is a bourgeois restaurateur who explicitly sees opera as the confirmation of her daughter Veda’s perfection (but is incapable of fully appreciating musical talent).

Opera is awfully appealing to some novelists and film directors, because it presents a constellation of ready-made themes that can act as a nifty subtext. “The Godfather Part III” used “Cavalleria Rusticana,” which is, like the film’s end, about revenge killings in Sicily. In “Quantum of Solace,” the action and music of “Tosca” were excerpted to back up the Bond plot about the evil villain and the female lead who kills him. And in “Mildred Pierce,” the Bellini aria “Casta diva” from “Norma,” an invocation to the Goddess by a supposedly virginal Druid priestess, accompanies the scene in which Mildred kisses her sleeping daughter on the lips as the song implores “spread on Earth that peace which you make reign in heaven.” (Had the producers chosen to continue the aria, they would have found an even more apt section of text when the priestess, who is actually so un-virginal that she’s secretly borne two children to a Roman officer, sings to herself and to him, “Return to me, and I will give you my heart.”)

See how the plot of the opera took over that last paragraph? Here, ladies and gentlemen, is the problem that film directors face when they try to stick a bit of opera in their movies. Opera has a way of taking over, of subverting the point you’re trying to make, of being hard to control, of doing what it wants.

Part of the issue is purely pragmatic. It’s well and good to decide that opera is a great symbol to use in your film. It’s a headache to figure out how to realize it — finding a theater, an orchestra, singers, sets, costumes and an audience. (Film directors generally seat their opera-going protagonists in a box or balcony, so they won’t have to fill the auditorium with actual warm bodies.)

Will you use an existing production (as “Quantum of Solace” did); design a production of your own (as was done for the Pavarotti flop “Yes, Giorgio”); use real singers or voice-overs? The “Mildred Pierce” producers licensed four tracks from Jo, and Evan Rachel Wood, who played Veda, learned them by rote well enough to go through the motions. But the result of all this is that opera becomes a hurdle rather than a moment of art — and that’s how it ends up looking on film.

Then there’s the problem of reducing an art form to mere subtext while trying to keep things true-to-life. Cain, “Mildred Pierce’s” author, knew his opera; his mother was an opera singer, he had ambitions in that direction for a while, and opera figures largely in several of his books.

But Cain, in the book, has Veda singing “La mamma morta” (“They killed my mother”) from “Andrea Chenier” — a most appropriate piece of wish-fulfillment for Veda’s character but not, unfortunately, a piece that a young coloratura would sing, as Pamela Koffler, the film’s executive producer, accurately pointed out when queried on the point by telephone. Director Haynes opted for “Casta diva” instead. “It has a kind of emotional peak that I couldn’t give up,” he recently told my colleague Ann Hornaday.

But “Casta diva” isn’t an aria that most young coloraturas would sing either (it was included over the protests of the film’s classical music adviser at Deutsche Grammophon). And Jo, Veda’s voice in the series’s other operatic selections, has never recorded it. The producers, therefore, had to use another singer: The voice you heard in “Casta diva” belonged to another setting star, Edita Gruberova.

In short, we have an aria that doesn’t quite fit the character, sung by a different singer, lip-synced by an actress who seems to have put all of her focus on the mechanics of how an opera singer might look and none into how a singer might interpret a character to create the effect onstage that she is supposed to be having. Rather than a realistic portrayal, we get what amounts to an expensive pantomime that we all agree to accept, suspending our disbelief, as a thrilling performance. The great promise of that first hearing of the “Bell Song” founders on the shoals of mere convention.

It’s a problem common to films about all kinds of art: Art becomes a Hitchcockian McGuffin, used for its symbolic value but presented in snippets that reduce it to the equivalent of a T-shirt bought in a museum store. The problem with “Mildred Pierce” is that the promise of that first teasing hearing of the “Bell Song” is never quite fulfilled. The film falls flat when it tries to add visuals to that strong aural impression.

One solution: “Mildred Pierce,” the opera. I’m sure someone is already thinking about it.