The first time I see Emma Forrest, she’s on stage at London’s Mayfair Hotel presenting the UK premiere of her debut feature film Untogether. She’s glamorous, confident and assured, as you might expect from a woman who spent her teenage years writing for national newspapers, published her first novel aged 21, and worked as a screenwriter in Los Angeles before marrying Australian actor Ben Mendelsohn, star of Ready Player One.

Forrest, 42, isn’t above showing her vulnerable side. The Untogether screening is to raise money and awareness for charity Bipolar UK. Diagnosed with the disorder, Forrest chronicled it in her 2011 memoir Your Voice In My Head, alongside her relationship with her late therapist. She’d planned to make it into a feature, with Emma Watson attached, but then it fell apart after endless unproductive meetings “where people would be concerned that the character wasn’t likeable enough”.

It was, she says, “a crappy feeling” after the catharsis of writing the book, but it spurred her on to make Untogether, a film about the trials of two Los Angeles sisters. Girls star Jemima Kirke plays Andrea, a writer and recovering heroin addict, who begins the film bedding Jamie Dornan’s doctor-turned-novelist. Her younger sis Tara (Kirke’s real-life sibling, Lola) is pulling away from her boyfriend (Mendelsohn) and towards a married rabbi (Billy Crystal) at the local synagogue. It’s full of what Forrest calls “icky, difficult, shameful feelings” about relationships, toxic or otherwise.

One critic described the film as an “un-rom-com”, a phrase Forrest likes. “I’ll go with that,” she says, explaining she’s a big fan of Nicole Holofcener, the writer-director behind such similarly “un-rom-com” gems as Walking and Talking, Lovely & Amazing and Enough Said. Holofcener became something of a “mentor” to Forrest, reading the script for Untogether and giving her notes, after they met at a party.

Forrest began the script after envisaging the final shot — no spoilers here, but it does feature a seminal Primal Scream track — and from that she worked her way backwards. For all the characters’ troubles, she wanted to make a positive story — particularly after Donald Trump was elected. “It just felt really important and good and helpful to me personally — that I was making something really hopeful because both those characters started off feeling hopeless.”

Another influence has only just become apparent to her in the past week. She was 12 when she went to see Tracey Emin’s Everyone I Have Ever Slept With 1963-1995, a tent featuring the names of those she shared a bed with embroidered on the inside canvas. “At 12, your heart is so open to be shaped because you don’t have your identity fixed yet, and in hindsight it really snuck in there,” she says. In life, she adds, “You have to go through that icky ‘everyone I ever slept with’ feeling.”

A sharer by nature, Forrest recently published in The Guardian a piece about writing her script, with a part for Mendelsohn, and then the trauma of shooting the film after they separated. “It was really difficult, it was really, really difficult for both of us,” she tells me, “but I believe in his work and he believes in my work. Then out the other side, we have such an unusually good post-divorce relationship that I think is rooted in our respect for each other’s talent. That’s a huge part of 
our connection.”

When we catch up, it’s two weeks after the screening and I find Forrest in particularly reflective mood. She stayed up late the night before watching the Panorama special on antisemitism in the Labour Party. “Bad, bad sleep,” she mumbles. She returned to London two years ago, after her marriage to Mendelsohn collapsed. “It’s weird obviously to be here; to be Jewish in England right now is to define yourself by what you’re resisting and what you’re fighting. This is endless, this bullshit.”

Born into a Jewish household in London — Forrest calls herself a “cultural Jew” — she was raised with younger sister Lisa into a comfortable middle-class existence. Her father Jeffrey worked as a solicitor while her mother Judith wrote for television. “If she’s an influence on me, it’s probably more as a character,” she says of her mum. “My whole family, I think I would be drawn to even if they weren’t my family… I find them interesting.”

In her early teens, Forrest wrote to Nigella Lawson asking if she could interview her for the school magazine. After sending Lawson the resulting piece, she got a call from the Evening Standard, who’d heard that Forrest “might be a good person to work with”. She reviewed Madonna’s Erotica for the paper, and soon afterwards was awarded a column in the Sunday Times.

It was a dream come true, though Forrest calls herself a “freaky teenager” for this stoke of luck.

“It’s weird. It’s not normal. I don’t think my writing was very good back then.” By the time she entered her twenties, she published her debut novel, Namedropper, leading her to be described as the “Jane Austen of the techno-generation”. Two further novels, Thin Skin, and Cherries In the Snow, followed, by which point she’d hot-footed it to Los Angeles.

Writing and selling scripts in Hollywood, Forrest was living the typical life of the screenwriter, being paid for stories that languish in development. It was frustrating, but — unlike freelance journalism —it gave her financial security.

On the inside, though, she was in pain. There was an attempted suicide and bouts of self-harming that left scars on her arms and other part of her body. Living in LA, she began to feel lost and realised that there had to be something else to life.

“That’s when I started going to synagogue on Fridays to this service called ‘Friday Night Live’” Based at the Sinai Temple, in Westwood, “it was just really amazing musicians —incredible jazz and gospel. It was like a gospel service.” She pauses. “I’m a music freak —they got me through the music.”

It was here that she filmed the synagogue scenes with the legendary Billy Crystal.

“We got really unlucky because we were actually shooting during the High Holy days, so it was hard to find a synagogue anywhere that would let us shoot,” she sighs. Thankfully, Crystal was a “pro” throughout, even when he insisted on re-working one sequence. “One of the most amazing moments of my life was to sit in a hotel room, look over and realise I was collaborating with Billy Crystal, making a scene funnier.”

I wonder if making Untogether brought her closer to Judaism. “I still don’t believe in God,” she says, “but I definitely believe in the power of anything that a large amount of people have believed in for a long enough time. I think as much about Judaism as I do about astrology. Both of them hold a power for me; both of them to me are rooted in the strength they give people and that people believe it and have believed it for thousands of years.

“I’m very moved by people believing. I don’t have to believe in God, but that they do is impressive to me. It’s really easy to be a non-believer.”

Since returning to London, where her daughter Carolina now goes to school, Forrest has yet to find a synagogue to go to, but she’s gradually finding her feet. “I have Jewish friends,” she offers. “I have a Jewish book club I’m in.” After two decades away, returning to the capital has “really fired my imagination”. Whatever her next film will be, she wants to shoot it in London, both for practical and creative reasons.

In the immediate future, she has her fourth novel due for publication in October. A “love story set during the wedding of Charles and Diane between two fans of theirs”, Royals does sound like a perfect homecoming book for Forrest to be writing. Will she turn it into a film?

“I’d love to,” she chirps. “That’d be lovely.” But maybe a play or a mini-series would be better. “If you have an idea you really care about, you have to think hard about whether making it as a movie is the right way.”

Somehow, you just know she’ll find her true path.

‘Untogether’ is available on digital download on iTunes, Amazon and Sky. ‘Royals’ will be published by Bloomsbury in October.