Federico Fellini cast a jaded, bemused eye on the postwar Italian decadence and indulgence with “La Dolce Vita,” a cinema classic about a generation just removed from decades of fascism and war partying, reveling in paparazzo-pursued celebrity and an internationally celebrated film industry.

Marcello Mastroianni played our tour guide, a seen-it-all journalist reduced to chasing and documenting the whims of shallow celebrities for a culture that couldn’t get enough of that piffle.

Modern Italian filmmaker Saverio Costanzo looks back on that era and casts events and attitudes in a more sinister light with “Finally Dawn,” about a starstruck young woman who samples early ’60s “la dolce vita” (the good life) when she’s cast as an extra in an American film production at the famed Cinecittà film studios.

Mimosa may be the “plain” sister, just tagging along to see that nothing untoward happens to her prettier, talent-scouted sibling Iris. But she’s the one plucked from auditions to play a “featured extra” hand maiden to an Egyptian queen, portrayed by American screen siren Josephine Esperanto.

As a day on the set spins into a night on the town with the star-who’s-taken-an-interest-in-her and her entourage, wide-eyed Mimosa (Rebecca Antonaci), trapped in a whirl of rich and perhaps depraved sophisticates, starts to fear for her safety and wonder if her fate is tied to another extra on that same film, an aspiring actress who turned up dead in the surf of a seaside Roman beach.

Where Fellini found bubbly cynicism, Costanzo hints at rape, drugs and murder. And in trying to turn suggestions of a thriller into “art,” he makes aesthetic choices that drown his melodrama in dullness.

A dour, sentimental black and white prologue sets the tone. It’s a downbeat war movie Mimosa, sister Iris (Sofia Panizzi) and their mother (Carmen Pommella) are catching on a Sunday afternoon at the cinema. “The Sacrifice” builds tension by showing us a mother and child hiding from a murderous German on the day of Rome’s liberation, a scene that unfolds ever-so-slowly and suspensefully. Shots are held well past their payoff.

And that “slow,” almost agonizing wait for a “cut” and moving on to the next scene becomes the editing strategy of not just the overlong sample of a film-within-a-film. It tediously takes over “Finally Dawn,” and rarely pays off as it does.

Brit star Lily James (“Yesterday”) plays Josephine, the regal star of this “swords and sandals” epic. But as we watch her on the one excruciating long take in which the champion from a foreign empire (Joe Keery) bests her hulking proto-gladiator and wins the right to her young stepdaughter (Rachel Sennott), we pick up something from Josephine’s stillness and her extravagantly coiffed and made-up scowl.

Josephine’s an entitled movie star, but a bad actress. Her performance brings to mind Elisabeth Shue’s orders in “Palmetto.” Being told to play someone bad at acting in a movie that’s not a comedy almost never comes off.

Mimosa makes eye contact with the star during that long take fight and negotiation. And as she tries to find her “just another extra” sister and locate their mother, supposedly waiting for them, as she shyly tries to avoid changing out of her costume in front of other women, a fancy dress and shoes are delivered to her. From Josephine.

When she finally exits the nearly empty hundred acre studio lot, who picks her up to take her home but Josephine, co-star Sean (Keery) and Josephine’s “one friend in all of Rome,” the art dealer Rufus (Willem Dafoe). But they’ve got a stop or two to make on the way.

Dinner devolves into “Let’s go dancing” which takes them to a hedonistic party where rare “snow from Bolivia” (cocaine) is sampled. Even though she doesn’t speak English, Mimosa picks up on Josephine’s reputation — insecure, on her third divorce and “completely crazy,” the crew assures her. The star’s conversation gives away pretension covering for vapidity.

Films like the lavish spectacle she’s starring in can be classics. And “If it’s classic, it’s not old. It’s eternal.”

But what’s her interest in Mimosa, whom she takes to calling “Sandy” and passing off as “a Swedish poetess” to her fellow revelers?

“You’re her passtime tonight.”

Warning Mimosa to “keep your guard up” seems unnecessary. We’ve watched her wander the vast backlot, with sets and soundstages and other productions and film folk gathered in a screening room to watch a newsreel about the murdered actress-wannabe Wilma, who died while working on this very film.

No film buff could mind Costanzo (“Hungry Friend” was his) taking his time leading Mimosa past the hustlers and callous casting combo that they procure would-be-starlets for, through the teeming backlot at Cinecittà. What we can dismiss is his confusion of slow long takes for art.

This isn’t “I Am Love” and he isn’t Luca Guadagnino. Sometimes “slow” is just your movie telling you that whacking twenty minutes culled from the end of every take that goes on too long wouldn’t be a bad idea.

The picture is pretty, but hardly lavish or lovely.

Antonaci is properly wide-eyed and guileless, Sennott her usual intimidating self and Dafoe deftly manages the charm that might be a mask for something uglier for his character.

James? She’s moved on, no doubt wisened to be wary of any filmmaker who sees her as a femme fatale, and asks her to be a bad actress in playing her.

Rating: TV 18+. drug abuse, sexual situations

Cast: Lily James, Rebecca Antonaci, Joe Keery, Willem Dafoe,
Sofia Panizzi, Alba Rohrwacher and Rachel Sennott

Credits: Scripted and directed by Saverio Costanzo. A Samuel Goldwyn release on Amazon.

Running time: 2:00