Of the five Oscar nominations for the coming-of-age tale Lady Bird, the most significant is the Best Director nod for Greta Gerwig, who is one of only five women ever nominated for the award in the history of the Oscars, and the first woman given the honour for her debut film.

The other female nominees are Lina Wertmuller for Seven Beauties (1975), Jane Campion for The Piano (1993), Sofia Coppola for Lost In Translation (2003) and Kathryn Bigelow for The Hurt Locker (2008), the last being the only woman to ever win.

Speaking at a screening of the film in Los Angeles late last year, 34-year-old Gerwig - who was previously known mainly as the star of Noah Baumbach's indie films - is, at first glance, as diffident and girlish as her on-screen personas sometimes are.

But as she talks about making Lady Bird - which opens in Singapore today and is also up for Best Picture, Best Actress (Saoirse Ronan), Best Supporting Actress (Laurie Metcalf) and Best Original Screenplay (Gerwig) on March 4 - she reveals herself to be an extraordinarily prepared film-maker and one who executed her vision for the film flawlessly.

The movie casts Ronan as a teenager, Christine, growing up in Gerwig's home city of Sacramento, California, which the girl considers to be a dead-end town.

The story then tracks her love-hate relationship with both the city and her mother, played by Metcalf.

Gerwig - the only female Best Director nominee at this year's Oscars - says her long career acting in low-budget indie movies proved invaluable when it came time to step behind the camera and helm her own film.

Her acting resume includes her Golden Globe-nominated turn in Francis Ha (2012) as well as Mistress America (2015), two of the low-budget, naturalistically acted "mumblecore" movies she co-wrote with writer-director Baumbach, who is also her boyfriend.

These and films she did with indie film-makers Jay and Mark Duplass (Baghead, 2008) and Joe Swanberg (2008's Nights And Weekends, which she co-wrote) were "a wonderful way to, on a granular level, understand how films are put together and what they want", she says.

"Because we were editing (Lady Bird) at night, mostly, and with the movies I made with Joe Swanberg and the Duplass brothers, the editing was happening at night or on our off time.

"And I would watch how the footage was put together - which makes you instantly not vain as an actor because you just become very rigorous about what you're doing, what's working and not working, and what is the shot you need to make the scene make sense.

"So some of that became my film school in terms of how to put something together."

And well before her first day on set for Lady Bird, which cost a relatively modest US$10 million (S$13 million) to make, Gerwig also did what a lot of first-time directors do: "I prepared and then I over-prepared to make this movie."

She explains: "Film-making is so odd because it's one of the only timed arts - when you're on set, every second that you have, every moment that you're there, is money. So every second that you spend doing one thing is a second you don't spend doing something else.

"And because of that, I spent so much time in the (preparation stage) with my cinematographer and my first assistant director and my entire design team. It was so much time shot-listing and storyboarding the entire script once through and once again when we had the actual locations."

She recalls: "We had such a clear game plan. We even know the lens parameters, like, here are the lenses we want to use and these are the sizes.

"(It was) all of this time I felt I could spend before someone was sitting there with a stopwatch and money was running.

"And the same thing was true of rehearsals," she adds. "So by the time I got (to set), I was nervous, but I was also ready. I was raring to go."

And directing was everything she imagined it would be.

"One of the greatest pleasures I've had is having actors saying my lines and a group of people figuring out how to make what I want come true.

"Because it's this simultaneous moment of supreme control - it's your words and your vision - and also a complete lack of control because they are the keepers of the words and ideas. So it's this odd thing of you're flying the plane, but the plane is flying you.

"It's something in the middle that I find totally intoxicating and I always have," she adds.

"And that feeling was so strong making this movie."