Greta Gerwig grabs the plumpest cushion in the room, kicks off her shoes and curls up on the couch beside me. The effect is not unlike being invited to a slumber party and, in that spirit, she trills through her current loves and crushes. She loves watching John Wayne onscreen. She loves Gene Kelly. She loves her own name: “It’s a bit old-fashioned, so you never meet many Gretas.”

She especially loves Colm Tóibín: "I'm reading Nora Webster at the minute. The writing! I love every page. So much."

It’s tempting to view the sometime actor, writer and director as a perfect marriage of sunny California and witty New York. She was born and raised in Sacramento but relocated to the east coast to attend Barnard College. A ballet dancer and fencer, Gerwig intended to do a degree in musical theatre but opted for English and philosophy.

"I loved musicals," she says. "I loved Singin' in the Rain and Oklahoma and Brigadoon. That's what I thought films were. Because growing up, my parents took me to every cultural event in Sacramento. I saw a lot of community theatre. We'd go to operas and ballets. There was a Shakespeare festival we'd go to every year. But we didn't have a rep cinema. Or a good video store. So it wasn't until I got to New York that I started appreciating film as an art. That changed everything."

Gerwig (32) made her screen debut in Joe Swanberg's LOL (2006) and was swiftly embraced as the queen of Mumblecore. She co-wrote Hannah Takes the Stairs (2007) and co-directed Nights and Weekends (2008) with Swanberg. She also worked with such core 'Core people as the Duplass Brothers and Ti West.

“I loved making those films. It was a great way to learn film-making. You do every job. You learn what works and what doesn’t. We wrote scenarios but we didn’t write scripts because we were really into very intense improv at that moment. I’ve moved away from that a little. I think there’s a lot of value in handheld camera and improv. It’s just I find it more satisfying to take a rigorous approach.”

Dating the director

Rigour could easily be a watchword for Gerwig's collaborations with director Noah Baumbach, after Greenberg and Frances Ha. (The pair have been dating and writing together since 2011.)

“We spend a lot of time working on the scripts that we write together,” she says. “Because we don’t do any improvisation or changing of lines. So the script is really the biggest part of the process. It’s the reason we make the movie. Because we think we have a pretty good piece of writing.”

She says she enjoys the two- headed approach.

“Collaborating, especially with good collaborators, is so much fun. It does help with the loneliness of writing. I’ve done both. And it makes sense with film. At a certain point all films are collaborative. You have to share with actors and producers and a DP. But having someone to share that moment when you’re just imagining is great.

“I still have trouble writing something that would never be spoken. I can’t think about something that’s just going to exist on the page. I have to be giving life to something whether it’s a play or a film. Maybe in ancient Greece I could have written poetry. But I don’t think there’s a big market for that anymore.”

Mistress America is Gerwig's third collaboration with Baumbach. The film concerns the friendship between a lonely college freshman (Lola Kirke) and her older, wackier stepsister-to-be (Gerwig). Gerwig and Baumbach's script maintains a lovely and delicate balance: we love its various screwball characters while deriving merriment from their foibles.

“We don’t think about making a character likeable or unlikable,” says Gerwig. “We just try to make them human. And I think – for me of course – humans are capable of doing terrible things. But in my experience of the world, most people, if you spend enough time with them, are quite complicated. People aren’t all one thing. So characters shouldn’t be one thing.”

Mistress America is populated by the kind of characters one seldom sees anymore. That didn't happen by accident.

Between tiny and titanic

"Right now it feels like there are tiny movies and superhero movies and nothing in between. We wanted to make a big character, but with psychology, not superpowers. We were thinking a lot about characters that existed in '80s movies like After Hours and Something Wild, where you have the idea of this woman – it's usually a woman – who is kind of underground and wild and a bit dangerous. She's charming but there's maybe something a bit unbalanced there. And she takes the uptown square person and drags them into craziness.

“It felt like that character had kind of disappeared from American cinema. I don’t know why. We wanted to bring that kind of hustler back.”

Mistress America suggests that Gerwig's Brooke is a lost American archetype. Almost like a cowboy.

"I do think the striving and lack of qualifications for the ambition that she has . . . that feels big and American to me. Even the title. Mistress America. It sounds like you're really going for something that might be the wrong thing. I liked that idea of misplaced ambition. And largeness."

Even Brooke’s walk seems larger than life, I note.

"She swaggers!" says Gerwig. "Except we slowed her down with heels. I was thinking about Eddie Murphy in Beverly Hills Cop. Apparently, he's doing his interpretation of Bruce Lee's walk.

“That’s such a great movie walk. I love good movie walks.”