So began a collaboration between Reitman and Cody that would bear fruit in 2011 film Young Adult and now Tully, about a mother struggling with postnatal depression after the birth of her third child. Charlize Theron stars in both movies, which reflect the life stages of the writer-director duo.

Reitman, 40, is the divorced father of a pre-teen daughter, while Cody, also 40, has three children with her husband, Dan Maurio, and lives in the San Fernando Valley area of Los Angeles.

“I felt like I eventually had to write the parental perspective movie and, as I am now raising three children, I did truly think it was the right time of my life to do that,” Cody says.

Reitman says of Cody, whose real name is Brooke Busey: “We’re in the midst of a lovely sine curve that will continue throughout or lives.

“I hope Brooke writes more scripts and writes them quickly because I love directing her movies. There’s nothing strategic about her writing. I think her process is more like a fever dream. It’s a pure birthing process that happens very quickly.

“An idea kind of works itself up and then she sends it to the page and it’s around six weeks from page one to the end of the script.”

Cody says: “Sometimes I feel like I don’t have the same creative freedom I had when I was younger. I think it’s harder the longer I do it. But the more life experiences I have amassed, the more qualified I feel to tell stories. So that’s one of the cool things about getting older. Maybe the only cool thing.”

She did not write the mother role with Theron in mind.

“I have a superstition. I never think about actors when I’m writing, because typically the person you have in mind is not going to be available or interesting. In this case, after making Young Adult together, I’d wanted to work with Charlize again. So when I heard that she was interested I was excited about that.”

Once cast, the actress transformed herself, as she had for her 2004 Oscar-winning role in Monster, gaining close to 23kg (50 pounds) to play a pregnant woman and then a weary nursing mum.

Crazy Rich Asians: ‘Eleanor is my mom’, director says of Michelle Yeoh role

The story follows her character Marlo, who accepts the invitation of her wealthy brother (Mark Duplass) to hire a night nurse called Tully (Mackenzie Davis). Tully cares for Marlo’s baby through the night and even becomes a confidante, giving Marlo the lift she needs to cope with the mayhem and to resume a happy life with her husband, Drew (Ron Livingston).

“It was something I’d never heard about growing up in the Midwest,” explains Cody. “After I moved to LA I found that when people have babies, particularly wealthy people, they pay somebody to come to their house overnight. The person shows up late, usually after most of the household are in bed and they leave at around six or seven in the morning.

“So often the night nurse is only having interaction with the nursing mother. I have actually encountered real-life situations where the baby’s father knew nothing about the night nurse, so I thought OK, I can have some fun with this.”

The film is part social commentary – showing how Marlo’s wealthy brother was “weirdly able to purchase temporary sanity for his sister”, she says – but with a twist Cody cannot reveal. Suffice it to say that Marlo lives in a kind of dream world brought on by the screaming baby and demanding children – which Reitman captures perfectly in a dizzying montage.

“I’d prefer to work with Jason at all times,” says Cody, whose only directing credit to date is for the 2013 film Paradise. “I don’t direct and it’s great that I can trust Jason with some of these crazy ideas.”

One of her crazy ideas is having Marlo dream about a mermaid in Tully. “The closest metaphor I could think of for the sleep deprivation you experience after you have a baby is being completely underwater,” she says.

“At the time I was going through that I was actually writing the script as a form of therapy, and I thought, ‘What if you’re underwater and there’s somebody under there to pull you to the surface?’ And that’s what Tully is.”

Reitman adds with a sigh: “Having shot two days of mermaid, I have no idea how James Cameron made The Abyss. Incredibly there are actual mermaids you can reach out to; one came to my house and swam in my pool, which delighted my daughter.”

Donnie Yen talks Mulan, Ip Man 4 and his passion project Big Brother

Reitman loves directing actors who are on his wavelength. He’s just worked with Hugh Jackman on The Front Runner, the story of 1988 US presidential hopeful Gary Hart, whose career was derailed after a scandalous love affair.

The film will have its world premiere in Reitman’s hometown, Toronto, at the city’s international film festival next week, and the director sings the praises of his Australian leading man. “Hugh is just incredibly decent and in a very real way. He’s legitimately just thoughtful. I kind of love that guy,” he says.

“I don’t rehearse,” he says. “I get to work with very talented actors and the joy for me is the process of discovery over the course of the day. We shot Tully in less than 30 days and thankfully Charlize feels exactly the same way. Rehearsing is the last thing she ever wants to do.

“You can feel it when you watch her work – and transform. It just happens before your eyes and that’s kind of a miracle.”

Perhaps Theron can pull off an Oscar nomination for her performance, even if there’s stiff competition in a year when women’s leading roles have been particularly strong. After all, there are none quite like this.

Tully opens on August 30

Want more articles like this? Follow SCMP Film on Facebook