“It was that weird experience that sometimes happens with actors,” said Lady Bird writer/director Greta Gerwig, pausing to remember, “where you genuinely feel like a third person entered the room.”

Gerwig was recalling a moment two years ago, when she first met with actor Saoirse Ronan (Atonement, Brooklyn). Though Gerwig had originally envisioned casting somebody unknown, she was intrigued upon hearing that Ronan had read and liked the script, and met with Ronan at the 2015 Toronto International Film Festival. It was just the two of them, reading the script aloud.

“It was like I got to meet Lady Bird for the first time,” said Gerwig, of Ronan’s characterisation. “I got actual goose bumps when it happened. She’d been living in my imagination and on the page, and all of a sudden she was there.”

Lady Bird, a senior at a Catholic high school in Sacramento, California, had been living in Gerwig’s imagination for a while. Though she has some parallels with her creator’s own life (Gerwig is also a Sacramento native who graduated from the city’s St. Francis High School 15 years ago), she is a fictional creation.

Lady Bird is a young woman who’s rejected her given name (Christine) for something more distinctive; who dyes her hair magenta and runs for student council though she knows she won’t win; who dreams of going to school on the East Coast (like Yale, but not Yale) and of one day “living through something.” She adores and is infuriated by her mother, and falls in love as if she’s hurling herself off a cliff but has as her ideal prom date her best friend, Julie. In short, she is entirely herself — and she’s, perhaps, all of us, long ago.

And she represents Gerwig’s first solo foray into feature-film directing, after a “film school” that consisted of acting in more than two dozen movies over the past decade, co-writing several and co-directing one.

The name Lady Bird has nothing to do with the former first lady; it arrived unexpectedly, when the script was in its very early stages. It came in the form of a line of dialogue — “Why won’t you call me Lady Bird? You promised me you would” — spoken by the character in the film’s first scene. Gerwig said the line just popped into her head one day, unexpectedly.

“The name Lady Bird was not something I had a particular association with. It just came out that way,” Gerwig said. “Later I remembered the Mother Goose nursery rhyme (‘Lady bird, lady bird, fly away home’), but it wasn’t a conscious memory. I think often your unconsciousness knows more than you do, it’s more tapped into something.” She was intrigued, she said, by the idea of “self-authorship, writing your own self as a character” that renaming represented, and also by the rock-star element of giving yourself a new name. “But I didn’t think of any of that when I wrote that line — it just came.”

Ronan, in the title role, headlines a strong cast of actors: Laurie Metcalf as Lady Bird’s mother, Tracy Letts as her father, Lucas Hedges (Manchester by the Sea) and Timothee Chalamet (Call Me by Your Name) as love interests; Beanie Feldman (remember that name) as her best friend; Lois Smith as an understanding nun. Gerwig made it a priority to “overprepare” so that once she was on the set, she’d have the flexibility to give the actors whatever they needed.

“I think the task of the director in a lot of ways is to hold the perimeter,” she said, “to give (actors) the space and the safety and the trust to do brave work. You create this magic bubble of safety.”

With Lady Bird scooping up year-end accolades (the New York Film Critics Circle named it the year’s best film, and it’s sure to be in the Oscar conversation), there’s no doubt that Gerwig will be directing again soon. She’s intrigued by the idea of regional filmmaking — of making movies outside of New York and Los Angeles, citing as recent examples Richard Linklater’s many films set in Austin, Texas; Barry Jenkins’ depiction of South Florida in Moonlight; Alexander Payne’s films in Omaha; Gus Van Sant’s in Portland. Lady Bird is in part, she said, a love letter to her hometown.

“I think in a way, directors are world builders,” she said. “They create worlds that don’t exist, and they exist for 90 minutes or two hours and then they’re gone again.” If they’re like Lady Bird, though, they linger for a long time afterward.