On Lady Bird’s first day of filming, Saoirse Ronan overheard a line that set the tone of the whole shoot. Newbie director Greta Gerwig bounced onto set, turned to the cinematographer, and grinned, “This is the happiest I’ve ever been in my life.”

Gerwig was in her hometown to shoot a personal—though not, she stresses, autobiographical—story about a Sacramento-born blonde (check) graduating high school shortly after 9/11 (check) who squabbles with her mother, a nurse (check), and dreams of moving to Manhattan (check). Half-a-million people live in Sacramento, ten times the population of Ireland’s County Carlow, where Ronan was raised, and yet her return felt like a reunion. Growing up, her friends shared the same doctor, the same dentist, and the same first job at an ice cream store. When Gerwig mentions a former neighbor, Ronan beams, “I met Rose!”

No wonder Lady Bird calls Sacramento, “The Midwest of California.” Once, while the pair shot a quick scene outside a bank, a car pulled up and hollered, “My mom said to tell you to tell your mom that she’ll see her at the med center!” Gerwig hadn’t seen the man behind the wheel in 16 years.

After she left home to become an indie film actress, Gerwig occasionally gets stopped at coffee shops in New York, a modest level of fame that reminds her of home. “It feels like an extension of being in Sacramento where everybody knows who you are,” says Gerwig. “I’ve recreated some level of community.”

Ronan was literally stepping into her director’s past life. The young actress never experienced the ordinary, all-American teenagehood: prom, college applications, parents anxiously filling out financial aid, continuous panic to figure out who you are and who you should be.

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“I didn’t go through any of that,” admits Ronan. She’s been acting non-stop since she was 9 years old, and got her first Oscar nomination for Atonement at 13. But she had Gerwig’s diaries, which Gerwig had even kept hidden away from herself while writing Lady Bird’s script. (“I tried to rely on memory,” explains Gerwig. “You remember the emotion far more than you remember the actual event, much in the same way you remember dreams.”) And more importantly, she had Gerwig’s trust. Two years ago at the Toronto Film Festival when Ronan was premiering Brooklyn, the film that earned her a second Oscar nomination, Gerwig invited herself up to Ronan’s room to read her the finished screenplay.

“I wrote a person I wasn’t sure existed,” says Gerwig—a dreamy, moody, creative, confident, fragile, self-destructive, romantic, pretentious girl, who throws herself from cars and shuns her birth name, Christine—“and then she started to bring her to life. Lady Bird was really this collaboration between the two of us. That character would not be that character if not for her.”

Still, Lady Bird isn’t one character—she’s dozens who shape-shift from scene to scene, depending on if she’s with her friends, her parents, her teachers, her first boyfriend, her second boyfriend, or alone with herself, wondering who she wants to be. She shrinks around the rich kids at school, but clomps into drama club auditions like a show pony.

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“She’s a pretty great musical theater actor,” says Gerwig.

Blurts Ronan, “Is she though?”

Gerwig and Ronan are barely a decade apart in age, but their dynamic toggles between best friend and big sister. Gerwig affectionately calls her star “Surshe”. On set, however, Gerwig was mom. Not like Lady Bird’s fictional mother, played by a phenomenal Laurie Metcalf, who wants so much goodness for her child that she makes them both miserable. More like the kind of mother who exists in the happy bedtime stories: positive, supportive, kind.

“She’s like a really good parent who gives you that discipline and structure for you to feel safe, but also the love and belief to go out and do it on your own,” says Ronan. The actress had just come off of an emotionally draining run playing the vengeful witch-hunting bully in Arthur Miller’s The Crucible on Broadway eight times a week, and Gerwig intuitively understood what she needed before each scene, whether it was quiet or a note or music or just time to figure it out. “I desperately cared about getting this right, for both of us,” says Ronan, turning to Gerwig. “It’s your first film!”

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“I had such unshakable faith,” beams Gerwig. “I just knew. I did feel like a mom in that way of, ‘I’m not confused about this. You can be confused about it all day. I’m not.’” Gerwig infused her entire cast and crew with that same whole-hearted admiration, and in return, the set blossomed into a harmonious Hallmark family. “It’s not all like, ‘Kumbaya,’” laughs Gerwig. “Everyone’s working their ass off. But within that, there’s so much mutual respect.”

For a first time filmmaker, Gerwig had a vision. An early flirting scene between Ronan and co-star Lucas Hedges had to be shot on a high ledge so their legs would swing like kids. Sometimes, she’d direct in metaphors, conjuring up the bliss of an imaginary yogurt commercial and saying, “So much of being alive is knowing that you’ll never get the yogurt moment.” A later scene where Ronan propositions her crush outside of a school dance wasn’t clicking until Gerwig and Ronan went for a walk. Suddenly, they realized the glitch. “We’d been thinking about this all wrong,” said Gerwig. “You guys are still children. And you’re pretending to be adults.” They tried the scene again and it worked. “As a director, taking a minute and figuring that out was such a gratifying moment.”

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The opening scene where Ronan caps a fight with Metcalf by throwing herself from a car also just worked from the first take. It was towards the end of the shoot, and the fictional mother and daughter had been screaming at each other for weeks. Ronan calls Metcalf her “sparring partner”. For an hour, maybe two, they drove through farmland near UC Davis and fed off the other’s energy, and the backstory they’d already created. “Just from the first take, it felt like it had such a foundation,” says Ronan.

Gerwig agrees, with a shudder. “Even when I hear that scene now, I still get goosebumps.”

As for the scene when Ronan loses her virginity to an indifferent Timothée Chalamet, the actress turns to Gerwig and giggles, “You found that really difficult. Timmy and I were fine.”

“I was a complete wreck!” Gerwig groans. “I became a dorky parent who needs to over-explain everything!”

It’s Ronan’s turn to give assurance. “We had you believing in everyone,” she says. “When you know your parent loves you, you’re like, ‘I can do anything!’”

This feature is part of a series of three cover stories celebrating Women in Hollywood for the 12/13 print edition of Deadline’s AwardsLine magazine. Click here to read our feature with Patty Jenkins & Gal Gadot for Wonder Woman. For our story on Dee Rees & Mary J. Blige’s Mudbound collaboration, click here