TORONTO -- Lady Gaga took acting lessons, meticulously learned her lines and perhaps was even too prepared for her cinematic debut in "A Star is Born."

But the pop singer said director and co-star Bradley Cooper helped pull her down to earth with an important acting lesson early in the shoot.

"He said something off script, and I just said the line I'd memorized," remembered the singer, born Stefani Germanotta, in a recent interview.

"I kept repeating the same line over and over again because I didn't know what to do."

It happened while shooting a key scene that establishes the undeniable chemistry between two characters from different worlds. Cooper plays Jackson Maine, an alcoholic superstar of country and rock, while Gaga is Ally, a small-time entertainer who's given up her dreams of stardom.

The two meet by chance after he strolls into a drag bar seeking a quick drink. He winds up watching her bring the house down with a performance of "La Vie en Rose" and dreams of her playing massive arenas to adoring fans.

"A Star is Born" starts with a flirtatious friendship of mutual admiration in the wee hours of the morning, but blossoms into a passionate romance. And it's during one of those early scenes when Cooper was riffing off his written lines with improvisational bits.

His approach left Gaga feeling lost on how to react. She repeated the same lines trying to stick to the script.

Cooper recognized her struggle, she said, and urged her to relax and let out her pent up emotions. That's when Gaga said she "threw the lines out the window" and began speaking from the heart.

"It really taught me something about being an actor," she said.

"You have to know the story you're going to tell. You have to know the lines. But at the end of the day, you have to be as honest as possible."

Those improvisational moments help "A Star is Born" stand out from its previous iterations, which include versions in 1954 with Judy Garland and James Mason, and 1973 with Barbra Streisand and Kris Kristofferson.

Cooper wanted to instil an elevated sense of sincerity across the production that distanced it from other Hollywood versions.

"The one thing that I begged of everybody in the room here is to trust me," Cooper said.

"Even if the movie sucked... they were going to be authentic no matter what."

For Gaga, that meant stripping away the layers that make her a global superstar known for vibrant costumes and colourful hits like "Bad Romance" and "Poker Face."

In a story repeated by its stars over the recent press tour, Cooper wiped away Gaga's makeup during the first screen test, a literal unmasking of her famous personas.

Gaga apparently took that suggestion to heart as she prepared for the role.

"I dyed my hair and took my makeup off a few months before we started filming to get into the character," she added.

She also reconnected with a younger version of herself, the firecracker performer who trekked around New York City trying to book small gigs in the early 2000s, years before she climbed the pop charts.

But the singer insists that despite parallels that viewers may find between her and the fictional character, there are many more differences.

"Ally has completely given up," she said.

"She does not believe in herself. She does not believe she's beautiful. She doesn't think she has what it takes."

Gaga insists her own personality is quite the opposite. She built her career on her own terms while rejecting pressure from record executives to get a nose job and sell her songs to other artists, she said.

Cooper admires that sort of headstrong determination in his performers. He said it's the passion and commitment Gaga brought to the role that helped deliver on his ambitions as a storyteller.

"On many sets you're not living it. You're acting the hell out of it, but you're not living it," he said.

"It's a very different experience."