"Are you going to have a baby alone as a woman?" she offers as an example. "Are you going to have one with your friend? Or with someone you hope is going to be your life partner – but do you believe anyone is going to be your life partner? For the rest of your life! Is that something we still can believe in?"

Sometimes we do, of course. Miller has been married to Oscar-winning actor Daniel Day Lewis for 20 years. Sometimes we do but change our minds – as Lewis had, marrying Miller without telling the girlfriend who was then living in his flat. So many choices: it's the burden of modern life. It is also the stuff of comedy in Miller's film Maggie's Plan, in which a gaggle of highly articulate people have children, swap partners and then wonder where the hell they went wrong.

"How do we live together? What does love mean? What does romance mean?" Miller continues. "That's perfect food for me for screwball comedy. There are so many possibilities for absurdity."

Maggie's Plan stars indie darling Greta Gerwig, who is her usual klutzy, charming self. Maggie is 30-ish and mildly bohemian; she works at an art school trying to set up graduates with business opportunities and is, in fact, brimming with plans. At first she is thinking about having a baby on her own with the help of a turkey baster and an old university pal, a hipster who has given up pure mathematics to make artisanal pickle, then she meets the somewhat older John (Ethan Hawke, 46), an academic at her college, he who asks to read and commment on the novel she is writing, and things proceed from there. The turkey baster is put on hold.

John is, of course, already married. His wife is a brilliant Danish academic called Georgette (Julianne Moore, 55), who is older again and whose career overshadows his own. Maggie decides she is rescuing him from a gorgon. Three years later they are married with a toddler, but things are not going so well for the couple.

"I think people bring different things out of each other. And you might not like the person you become," Miller says. And so Maggie hatches a new plan: to get her husband and his ex-wife back together. There isn't much real pain here; the film skims gracefully over what results, because romantic comedies don't really do unpleasantness. No one is condemned by the film, either: neither the narcissistic John nor the self-interested Maggie. Gerwig sees Maggie as someone who just wants to live truthfully; it was her idea to make her a Quaker who, when she needs inspiration, slips into a Society of Friends meeting house.

"I think she wants to live her life with integrity and I think that for her means different things at different moments," she says. "I think she is actually comfortable with the fact that this is never going to be just one thing forever. That it's going to change."

Miller is quite sharp, however, about the politics of mother-work. The two women look after John's various children; he dips into parenthood when he isn't busy with his eternally unfinished novel.

"A friend of mine described motherhood as a game of tag where somebody tags you and you suddenly realise you're 'It' for the rest of your life," Miller says. "There is a sense of responsibility women tend to have and are expected to have. Don't get me wrong: that does carry a lot of privileges you get in the relationship you have with your children. But I did want to go into the funny aspects of that phenomenon."

Even so, she lets John live happily ever after. There is no real animosity between the women who do all this caring, either; the old romcom rules of jealousy don't apply.

"One thing that really struck me about this story was the relationship between the two women," Moore says. "They are not adversaries. When they finally meet, Georgette is very welcoming to Maggie and says 'you know, my kids like you a lot and you have been great with them'. And that is something I see a lot; I've had a lot of women say to me 'you know what, I like her, the kids like her and we're working it out'. As families get more complicated, I think they're making more effort to get along."

Julianne Moore has been married to director Bart Freundlich since 2003. What's their recipe for success? "I don't know. One day at a time, I think," she says.

She also says she finds him "very, very funny", which may be another helpful hint. Freundlich is 10 years younger than she is, as John is a decade younger than Georgette; Greta Gerwig is muse and collaborator with director Noah Baumbach, 14 years her senior, who left his older wife Jennifer Jason Leigh for her. Real lives – their lives – resemble at least some of what we see heightened and satirised in Maggie's Plan.

And when asked, they all say they love Woody Allen's films, especially Annie Hall. That figures, absolutely, but Miller isn't just shining up an old New York story.

"What she has done is slyly subvert a ubiquitous paradigm. She turns a standard male fantasy inside out," wrote critic A.O. Scott in The New York Times. "The movie, after all, is not about John's self-esteem. It's about Maggie's plan."

It's actually about what a woman thinks she wants. That does put a new spin on things.

Maggie's Plan is in cinemas now. 

And around we go: partner-swap movies

Private Lives (1931, directed by Sidney Franklin)
Adapted from Noel Coward's racier theatrical play, in which a divorced couple rekindle the fire while on honeymoon with their respective new spouses. The "comedy of remarriage" subsequently became a significant screwball sub-genre: see also It Happened One Night (1934), The Philadelphia Story (1940) and The Lady Eve (1941).

Romuald and Juliette (1989, Coline Serreau,)
Daniel Auteuil stars as a troubled executive who, after discovering his wife's affair with his own immediate underling, takes refuge with the office cleaning lady and her five children by different fathers. All are finally reunited as a very large, happy and multi-racial family. Frothily French, but with a serious anti-racist subtext.

Hannah and her Sisters (1986, Woody Allen)
Mickey is married to Hannah, who used to be married to Elliot (Allen). Mickey is also involved with Hannah's younger sister Lee. Elliott starts dating Hannah's older sister Holly. Everything works out thanks to (a) therapy and (b) a hearty Thanksgiving dinner. It's a New York thing.