Two years ago, screenwriter Diablo Cody was in the middle of trying to write "a very big, commercial, four-quadrant movie" when she gave birth to her third child.

After her first two kids, she had gotten back in the groove of work, but this time, shattered by exhaustion, she felt herself floundering both as a writer and as a parent.

"I was in this complete haze," Cody remembers. "It was like I was disconnected from reality. All I could do was fantasise about having somebody come and pull me out of this underwater wreckage I found myself in. I thought, I wish somebody would just show up on my porch tonight and just take over everything so that I could take care of myself."

Cody, 39, had always been dismissive of the idea of hiring a night nanny, thinking, she says, "the least I can do is wake up with my baby". But this time she decided to enlist someone to help her over the hump of sleep deprivation. The experience was revelatory.

"I was like a different mom in the morning," she says. "I was actually able to take care of my other children. I was energetic and chipper and patient."

In Tully, Charlize Theron stars as Marlo, an overburdened middle-aged mother of three.

READ MORE:
* ​Tully: Charlize Theron shines in drama about the minutiae of motherhood
* Charlize Theron 'lucky' she wasn't given lead in stripper flop Showgirls
* Charlize Theron: 'I don't care about ageing'

The big studio project fell away and the idea for another, more personal story began gestating in Cody's mind -- an idea that would eventually grow into the new film Tully. "I thought, imagine if an ordinary woman who doesn't live in a world of night nannies and household help – what if somebody stepped in and gave her that gift?" she says. "How would she transform?"

In Tully, Charlize Theron stars as Marlo, an overburdened middle-aged mother of three whose life is turned upside down in unexpected ways when her wealthy brother (Mark Duplass) offers to hire a young night nanny (Mackenzie Davis) to help care for her newborn daughter. Deliberately discomfiting and shot through with dark humour, the film marks Cody's third collaboration with director Jason Reitman following the 2007 teen-pregnancy hit Juno and 2011's darkly comic Young Adult, which also starred Theron.

Diablo Cody says her primary aim in writing Tully was to take on that "myth of the supermom" by drawing from her own personal struggles as a parent.

Even as she was writing Tully, Cody was well aware that a film that portrays a mother who is barely holding it together – and that tackles uncomfortable questions of how class intersects with parenting – threatened to touch a third rail in movies.

"In TV, I've definitely noticed that people are more willing to embrace the flawed female protagonist, but there's still something about the mother," she says. "We can deal with an unlikeable woman who's freaking out. But if she's freaking out on children, we're out."

For evidence of the risks, Cody needed to look no further than her own résumé. While Juno proved a runaway hit, grossing US$231 million worldwide and earning Cody an Oscar for original screenplay, Young Adult – whose central character, an acerbic alcoholic divorcée, proved off-putting to many viewers – failed to connect with a large audience. The 2015 dramedy Ricki and the Flash, which Cody wrote and which starred Meryl Streep as an aging rock musician, got a lukewarm reception.

"The feedback that we got from test audiences was that Ricki was a terrible person because she had chosen her music over her children," Cody says. "People do not like the image of a mother who is anything but flawless and nurturing."

Still, as raw and edgy as Cody's script seemed, when Reitman first sent a copy to Theron, the actress immediately connected with it as a single mom of two young adopted children.

"I had just come out of my dark tunnel," Theron says. "My little one was six months old and she had just moved out of sleeping in my room and was slowly starting to sleep through the night. So I was just coming out of that."

In a culture that often expects women to present a soft-focus image of maternal perfection to the world, Tully star Charlize Theron knew a film delving into the emotional minefield of motherhood had the potential to strike a chord.

In a culture that often expects women to present a soft-focus image of maternal perfection to the world, Theron knew a film delving into the emotional minefield of motherhood had the potential to strike a chord.

"We're so honest about everything – we share what medication we take, what happens in our bedrooms – but we don't talk necessarily about this stuff," says the actress, who gained nearly 50 pounds for the role.

"I had experienced it myself with my own kids. I definitely had moments where I felt complete judgment, and I found myself feeling a little bit ashamed for not doing it all myself or for having somebody tell me that I'm screwing my kids up by giving them formula. So [reading Cody's script] felt like a relief. I was like, 'Oh, somebody else is feeling this as well'."

Charles Sykes/AP

Charlize Theron gained nearly 50 pounds to play Tully's Marlo.

Both as the father of a young daughter and as a filmmaker drawn to flawed, messy characters, Reitman was similarly excited by the prospect of probing some of the darker corners of child-rearing.

"It opened up conversations about our experiences being parents," he says. "One of the many things that no one talks about it is not knowing what to do, being scared to ask because you're supposed to know what to do and how lonely that can feel."

Cody says her primary aim in writing Tully was to take on that "myth of the supermom" by drawing from her own personal struggles as a parent.

"I will never be that mom who makes the Minions cupcakes for the class party – ever," she says. "I feel like women are expected to be great at the nurturing aspect of things, and that didn't come naturally to me and that was scary. People talk about how motherhood transforms you, and it does in a sense. But in another sense I'm still the exact same impulsive, selfish knucklehead I've always been. I expected to wake up and be Mother Teresa, and that didn't happen."

But Cody believes that the film, which has received largely positive reviews since its debut at the Sundance Film Festival, will resonate with many mothers who rarely see the challenges of parenthood dealt with honestly onscreen.

"I've had so many moms reach out to me who've only seen the trailer, saying, 'I've been waiting for this movie'," Cody says.

Having dredged Tully out of her own most difficult moments as a mother, Cody is now eager to see how the film's story – which unfolds in unexpected ways the filmmakers have been careful not to spoil – will resonate with parents and non-parents alike.

"I want this movie to be ultimately uplifting and hopeful and I think it does end on an optimistic note, but I also wasn't afraid to go deep with it and go to a dark place," she says.

"It's the type of movie that you want people to discover and interpret. You don't want to give everything away."

Tully (M) opens in New Zealand cinemas on May 10.

 - Los Angeles Times