Margot Robbie and Diego Calva knew as soon as they dove into “Babylon’s” audacious screenplay that they wanted the “roles of a lifetime” in Oscar-winning filmmaker Damien Chazelle’s grandiose and destined-to-be-talked about silent-film era epic.

But both couldn’t help but wonder how anyone could ever pull off such a raunchy, over-the-top spectacle — and convince a major studio to financially back a 3-hour-plus film with hundreds of extras and an array of lavish, period sequences without the use of CGI.

“I felt like who is really going to be able to make this happen?” Calva recalls. “Damien was saying ‘I would like everything for real.’ I was like this is madness. It’s not going to be like that.”

Turns out, it was exactly like that. And it worked.

Calva, the Mexico CIty-based actor and director, is poised to make a big splash with his first major Hollywood role playing Manny Torres, a romantic Latino go-getter immigrant who gets swept into the hyperactive Wild West 1920s-era moviemaking turnstile. In this fever-induced, make-it-up-as-you-go industry, he encounters another dreamer, the mercurial, tormented actress Nellie LaRoy (Robbie).

The hard-R-rated “Babylon,” opening Dec. 23 in theaters, also stars Brad Pitt as alcoholic matinee idol Jack Conrad, Jean Smart as gossipy newspaper columnist Elinor St. John and Jovan Adepo as Black trumpeter Sidney Palmer, to name a few.

Early in December, Calva and Robbie blitzed their way through the Bay Area where Robbie received the Maria Manetti Shrem Award for Acting from SFFILM. Chazelle was slated to present the honor but had to cancel since he and his producer wife Olivia Hamilton were expecting another baby around that time.

Robbie, the Oscar-nominated star of “I, Tonya” and “The Wolf of Wall Street” as well as the upcoming feminist-tinged satire “Barbie” from director Greta Gerwig, said she shared Calva’s sentiments as she read Chazelle’s screenplay.

“My first reaction was I have to be a part of this,” she recalls. “For me it felt like ‘The Wolf of Wall Street’ and ‘La Dolce Vita’ had a baby — the wheels had come off.”

That love child description fits ever so well since Chazelle, the cinematic wunderkind who is the youngest director (at 32) to collect an Oscar for 2016’s “La La Land,” is a huge Fellini fan. “Babylon’s” jaw-droppingly decadent opening party sequence harkens to the debauchery that unfolded in Fellini’s influential 1960 classic.

Chazelle had been researching this dark Hollywood tale for some 15 years before putting it to film.

“Right from the onset, I found myself fascinated by stories that I read about the fallout of the transition to sound in the late ‘20s,” he said from Los Angeles. “Stories of actors killing themselves. Stories of people putting guns to their heads, of drug overdoses, and the spate of suicides that swept through Hollywood. There was something that gripped my imagination like, ‘Wow, how could things have gotten this dire?’” (The film pays homage to the sweeter version of the story told in the classic 1952 musical, “Singin’ in the Rain.”)

Though the film references several actual events, Chazelle chose not to focus on anything specific.

“It’s like I had a growing collection of anecdotes or people from real life who inspired me or I was fascinated by and if anything they became the seeds for people in the movie.”

And while the scope of the movie was huge, the reality was he needed to keep to a relatively low budget ($78 million), which required a lot of prep-work for the 70-day shoot.

That was the longest time Chazelle’s ever had for filming, but given the broad reach of the story Chazelle felt — as he did with 2014’s “Whiplash” with Miles Teller and J.K. Simmons — he and the cast and crew needed to “hit the ground running” every day.

“We’ve just got to be running and gunning every day. It’s just got to be high energy every day and in some ways I think that it’s kind of appropriate to the spirit of the movie.”

For Robbie and Calva that on-set momentum Carried into their days off.

“During the weekend we were supposed to rest, but I was afraid I would get tired,” Calva said.

Says Robbie, “You know when you get sick after you finish a job? I was so scared that if I slowed down on the weekend I would completely fall to pieces.”

“I said I thought after doing ‘Wolf of Wall Street’ that I would never be on a set like this again and then this role came along 10 years later,” she adds.

But this experience might never be replicated again, Robbie laments.

“A lot has happened in the last couple years and I don’t know if anyone is ever going to be on a set again with 100 extras (with) practical explosions and an orchestra playing. That size it’s just too economical with CGI. You have to have someone like Damien who is hellbent on doing it all for real and all practically and to have the weight behind him so people actually say ‘OK we have to do it the way he wants to do it.’”

Paramount Studios, Chazelle said, “was amazingly supportive, more so than I thought any studio would be of the vision of the film and never trying to tame it or anything.” The hardest part was sticking to that budget.

Robbie knows about having to make compromises to get films done, she and her husband Tom Ackerley and two others, founded the production company LuckyChamp Entertainment, which produced “I, Tonya,” “Birds of Prey,” “Promising Young Woman” and others.

“The way that I felt on set as an actor, there were no compromises,” she said. “It never felt like we held back in any way. I’ve done films where I play characters that I’m kind of constrained due to a PG rating — like I’m not saying what my character would actually say right now. I’m not acting authentically. I’m controlling myself and I’m putting shackles on my character right now for a certain rating or certain plot points. That didn’t feel like it existed in this movie.

“I felt like Nellie was off leash and I was off leash. But really it was the first time in my whole career (where) I felt like there were absolutely no boundaries.”