Eliza Scanlen in the spotlight again in a 'delightfully bent love story'

It's a blast-furnace summer morning in the northern suburbs. Inside a low brick house, shrouded in black plastic to shut out the daylight, Ben Mendelsohn and Essie Davis confront an intruder in a scene from the new Australian film Babyteeth.

They play a quirkily mismatched couple, psychiatrist Henry and former concert pianist Anna.

Eliza Scanlen and Toby Wallace in Babyteeth. Credit: Lisa Tomasetti

The intruder, Moses, is a 23-year-old rats-tailed, tattooed drug dealer who has taken up with their gravely ill 16-year-old daughter Milla and is discovered rifling through the kitchen drawers in the early hours. He is played by Toby Wallace from the TV version of Romper Stomper; she by Little Women's Eliza Scanlen.

"You threatened my wife with a meat prong?" Henry says.

While the parents argue about whether to call the police, Milla wants Moses to stay for breakfast.

"I was dreaming about you when you came in," she says to him.

For the parents, the problem is apparent: their vulnerable daughter has fallen in love.

As that scene suggests, Babyteeth balances delicately on the edge of intense drama and eccentric suburban comedy, revealing emotional pain as it goes along. In two visits over a fortnight, everyone on set has a different way of explaining what the film is about.

It centres on Milla's cancer diagnosis and how it affects the lives of everyone around her, including her parents who are struggling in their own lives, her inappropriate boyfriend, her lovesick music teacher and a kooky pregnant neighbour. It is director Shannon Murphy's first film, having come up through theatre then such TV shows as Offspring, Rake and the SBS miniseries Over The Ropes, then heading off after the shoot for two episodes of Killing Eve.

While Scanlen has had more publicity than she would want lately – with her prize-winning short film Mukbang accused of cultural appropriation then staunchly defended by 27 leading filmmakers after screening at the Sydney Film Festival – all four central actors shine in Babyteeth.

Sitting under a gum tree in Sydney's St Ives over lunch, Mendelsohn raves about the film's distinctiveness. "It's a delightfully bent love story, it's beautifully Australian and it's got a heart the size of the Simpson," he says. "And it loves its people with all their weird messed-up bits."

Bringing Babyteeth to the screen started in 2012, on the opening night of Rita Kalnejais' play of the same name at Belvoir Street Theatre.

Eliza Scanlen in Babyteeth. Credit: Lisa Tomasetti

In the audience were acclaimed producer Jan Chapman, whose credits include The Piano, Lantana and The Babadook, and Alex White, her former long-time assistant. They were looking for a film to produce together.

"Jan and I were sitting on opposite sides of the theatre and we made a beeline into the Belvoir foyer and went, 'Oh, my God, this is it'," White says. "I'd known Rita for a long time as a friend so I knew she was writing it and I love the theatre so I'd been tracking it for a while."

What she admired was that the play was "completely irreverent, bittersweet, with a love story and such beautiful family themes ... Rita's got a really great sense of humour, then you're in tears. So it was just a completely surprising experience to walk out of the theatre feeling so alive."

They asked Kalnejais to adapt the play for the screen, with Richard Roxburgh down to direct at one stage before Murphy signed on, impressed by its originality.

"I'm such a stickler for being attached to material that's genuinely original and that's so hard to come by," she says. "When you try to describe Babyteeth, it sounds like a bunch of films you've already seen but it's just not. Tonally, it's so different and I found that a massive challenge."

Chapman calls the film "a fresh insight into families" that is bold and funny. White calls it a love story about a girl whose gift to everyone around her is "to help them feel".

Murphy has the catchiest description: "It's a story about how good it is not to be dead yet. It's an emotional story but it doesn't wallow in sentimentality."

Essie Davis and Ben Mendelsohn in Babyteeth.Credit: Lisa Tomasetti

There were changes from the play – one is that Milla goes from 14 to 16 – but Murphy decided, when she finally read the play, that she wanted its quirky chapter headings to feature in the film. They break up the story into scenes with such titles as Nausea, When Milla brought Moses home to meet her parents and What the dead said to Milla.

Casting Davis as Milla's mother was an easy choice, with Murphy wanting to work with her since seeing Steven Soderbergh's play Tot Mom at the Sydney Theatre Company in 2009. "She was on that massive screen and I remember just going 'who is that woman?'" she says. "She's extraordinary."

When Murphy's actor husband Dan Wyllie suggested Wallace, who worked with him on Romper Stomper, she was sceptical. "I don't listen to him most of the time," she says.

But Wallace's generosity and vulnerability while auditioning won the role and he went on to be named best young actor when Babyteeth had its world premiere at the Venice Film Festival last September.

Not so easy was who would play Milla, given she has to pass for 16 and be capable of wrenching hearts.

"We've got so many amazing young actresses in this country but it was actually really difficult to cast because she does so many different things in the story," Murphy says. "She's is a chameleon, constantly shifting and redefining who she is."

While she saw Scanlen early, Murphy kept auditioning other actresses.

"As a first-time filmmaker, I was like 'oh my God, I can't just cast the first person I've seen' but I came all the way back full circle to her," she says. "[Casting director] Kirsty McGregor was really good at guiding me back without making me feel like an idiot."

While Sydney-raised Scanlen's career was taking off in the US, firstly in the series Sharp Objects then being cast in Little Women, she wanted to work in Australia as well.

"It's just a really beautiful story and I love how Rita kept the theatricality of the play in the screenplay," she says. "It's an honest look at how young people suffer with cancer and also it's a celebration of life. And it's about a girl growing into herself."

Mendelsohn was harder to lock in for the film, given a thriving Hollywood career that has included Rogue One: A Star Wars Story, Ready Player One and Captain Marvel as well as an Emmy for the TV series Bloodline. "You have a significant role in a Star Wars film, things are going very well," he concedes.

Murphy knew she definitely wanted him when she went to a Sia concert.

"I'm such a stickler for being attached to material that's genuinely original": director Shannon Murphy.Credit: Joke Schut

"She had done this thing in her concert where she's got famous people in each music video," she says. "He was in this beautiful one – this amazing dance of destruction and pain. It was so moving that I was just standing there crying and going 'he's got to play Henry. I've got to find a way to make this happen'."

Murphy emailed him to say how moved she was by his Sia performance and how good it would be to return to a comedic role in Australia after darker roles in the US. When they spoke, Mendelsohn surprisingly said he loved the script and was in.

"It was like that was the easiest phone call of my life," Murphy says. "I'd meditated for half an hour beforehand so I didn't lose my shit on the phone and he was just so easy."

The shoot turned out to be "stifling and intense" over five hot weeks.

"We had to shoot Ben out [so he could return to the US] so we had to do all the very big meaty intense scenes up front," Murphy says.

"But then I had the whole second half of the shoot just with Toby and Eliza. It suddenly felt like we were making almost a different film because we got the time to just ease into their characters and their beautiful love story."

That beautiful love story saw then 20-year-old Scanlen show her commitment to the film by having her head shaved.

"It's been so liberating," she says outside Sydney Nursing School in Camperdown that is doubling for a hospital two weeks later. "At the beginning of the shoot, I had my hair cut short into this mangy look. Then there was one afternoon when we were shooting and I had to get it all shaved off.

"I was a little nervous but to be honest I felt like a new person. And it's amazing to swim with a bald head."

Babyteeth opens on July 23.