TORONTO — When Bradley Cooper saw Lady Gaga perform “La Vie en Rose” at a fundraiser at the home of entrepreneur Sean Parker, it wasn’t one of the important moments along the road to making “A Star Is Born.” It was, Cooper says, THE moment.

“She demolished the room,” he recalls, still wide-eyed about it. “I knew that was plutonium.”

The next day, Cooper went to Gaga’s home in Malibu to confirm that what he had seen the night before was real. He arrived hungry. Gaga – whose friends call her by her real name, Stefani – fed him some leftover spaghetti, and the two East Coast, Italian American-raised performers (Cooper is from Philadelphia, Gaga New York) felt an immediate, natural connection. “Instantly,” says Gaga. “When I saw his eyes, when I opened the door.”

Bradley Cooper, second from left, cast member and director of the film “A Star is Born,” poses with actors, from left, Anthony Ramos, Lady Gaga, Dave Chappelle and Sam Elliott at the Four Seasons Hotel during the Toronto International Film Festival in September. Associated Press/Chris Pizzello/Invision

Within minutes, they were singing by Gaga’s piano and “A Star Is Born” was, well, born.

“And when I heard him sing! My God! I stopped playing the piano and I was like, ‘Bradley, you can sing!”‘ said Gaga, sitting next to her co-star and director. “And he was like, ‘Really?’ And then he said, ‘Let’s film it.’ He started filming it on his phone.”

Cooper shakes his head. “It was nuts.”

It can be hard to separate the already mythologized transformations – Cooper directs! Gaga acts! – that fueled “A Star Is Born” from the fictional fable of fame, itself. In both the movie’s creation and in the finished product are lessons of bold chances and artistic integrity, of personal frailty and popular success. “A Star Is Born” is a movie mirrored by its making.

“A Star Is Born,” which opened Friday, is the fourth version of the story (or fifth, depending on how you count). First was George Cukor’s “What Price Hollywood?” in 1932, followed by William Wellman’s 1937 remake. Later came one with Judy Garland and James Mason in 1954 and one in 1973 with Barbra Streisand and Kris Kristofferson.

A new “A Star Is Born” has been in development for about two decades at Warner Bros., with various incarnations once planned around Will Smith and Whitney Houston, or Beyonce and Leonardo DiCaprio with Clint Eastwood directing. Cooper, who starred in Eastwood’s “American Sniper,” first discussed acting in the film for Eastwood before deciding to direct, too. For encouragement, Eastwood visited the set on the first day of shooting.

“I remember he said he liked my boots,” says Gaga. “I turned bright red.”

Cooper, though, put his own imprint on “A Star Is Born,” retailoring the story and – he hopes – launching himself as a writer and director. With meticulous preparation, Cooper – ever the student – threw himself into the new role. Often, he could be found under a table in a scene with a monitor so as to be as close as possible to the actors. “He was tireless,” says Sam Elliott, who plays Cooper’s brother in the film. “He never quit on it, from beginning to end. It probably drove the studio nuts at some point that he wouldn’t quit on it.”

“Being 39 when I started this journey, I just realize: Time is the biggest currency. If I don’t do what I keep feeling inside, constantly seeing shots in my head,” Cooper says, trailing off. “I always knew that at some point I had to stop critiquing other movies and just make one.”

Cooper stars as Jackson Maine, a hard-drinking, country-rock ‘n’ roll star in the vein of Gregg Allman. (Maine’s band is played by Lukas Nelson & Promise of the Real, Neil Young’s regular backing band.) When Jackson ducks into a drag bar for a drink, he’s blown away by Ally (Gaga), who’s there singing – what else – “La Vie en Rose.”

A naturally talented singer who has essentially given up on her music dreams, Ally has always been told her look (and her nose) isn’t quite right. She and Jackson quickly fall in love, even as Jackson’s drinking problem worsens, but not before they can together forge something honest and beautiful through music, catapulting Ally to stardom.

“When I’m watching it back, I see myself as a much younger girl, more like when I was 15 writing songs at the piano,” says Gaga. “What Jackson is trying to teach her is something that I still want to give more of in my music now and in the future. It’s the nakedness of talent.”

A rare fervor has greeted Cooper’s “A Star Is Born.” Its trailer has been watched more than 10 million times, many of them repeat, misty-eyed views. And if there’s one thing that accounts for its swoon-inducing power it’s this self-empowering message of fame coming to those who are true to themselves.

It’s something that resonates for many in the cast, too, like Anthony Ramos, who plays Ally’s best friend. The 26-year-old actor, who grew up in the projects of Bushwick, Brooklyn, caught his big break when Lin-Manuel Miranda cast him in “Hamilton.”

“I’ve had teachers tell me, ‘You have to be this or you have to be that to be successful. You have to change the way you speak. You have to grow your hair out.’ I’ve had people tell me all kinds of things to give me the formula for success,” says Ramos. “But what I realize, which you find by the end of this movie, all you gotta do is love yourself and believe in yourself, and continue to be your truest self.”

For even the 74-year-old veteran actor Elliott, “A Star Is Born” has been cause for reflection on his own path. For decades, Elliott, with his sonorous drawl and trademark mustache, has been resolutely himself, in any role.

“Nobody’s ever going to confuse me with a chameleon. I’m just not one of those kind of actors,” says Elliott. “Ben Johnson told me one time: ‘I might not be a very good actor, but nobody else can play Ben Johnson better than I can.’ And that somehow resonated with me. It was about character. It was about integrity. It was about what makes up the man.”

Gaga, who has rallied her fans (“little monsters”) around a message of self-acceptance, says she identifies equally with her character and with the more troubled Jackson. The pop star has previously been forthright about her struggles with mental health, and has said she was raped at age 19.

“Jackson’s plight in the film and his substance abuse, it really stays with me. The mental health aspect, the substance abuse aspect, the trauma aspect. I told Bradley right after we watched it in Venice that I had to take 30 minutes to myself in a back room somewhere,” says Gaga. “If I act again, the experience has to be as deep as this one or it wouldn’t be fulfilling to me.”

Cooper, too, says “A Star Is Born” has altered him.

“I find myself thinking of lines Jackson says often, just in terms of taking on a new project: What am I trying to say and how am I going to say it?” the 43-year-old says. “Any other project that comes after this, I just have to be brutally honest with myself and listen to Jackson.”

Ramos’ success recently inspired his own older brother to – like Ally does in the film – quit his job and “go for it.” With “A Star Is Born” now in theaters, more walkouts may follow, more stars ready to be born.

“Everybody quit their job!” jokes Ramos. “Naw, not everyone can quit. We need some people working.”