Writer-director recalls her childhood with a bipolar father in Infinitely Polar Bear

When her directorial debut Infinitely Polar Bear premiered at the Sundance Film Festival on January 18, 2014, Maya Forbes was surrounded by family: her mother was there; so were her younger sister (the singer-songwriter China Forbes), her father's three siblings, and her mother's brother.

Taking its title from her father's bipolar disorder, Forbes' autobiographical film recalls the childhood episode in 1970s when she and her sister went under the care of the household's loving but erratic patriarch, while their mother attended a business school in New York for 18 months, only visiting their Boston home on weekends.

"They loved the movie, so that's nice for me," Forbes says. "I was worried that they would think I hadn't captured my father and represented things correctly, [but] all the people who kind of knew and loved my father felt I really captured his essence."

And what would her father - who died in 1998 - say if he had a chance to see the film? "I know he would be embarrassed. 'Oh god, who cares about me? Why would anyone want to see this?' But I think he'd like it," says the 46-year-old, who finished her script in 2008 and felt compelled to direct it herself.

"Directing was what I'd always wanted to do, and the script got me over whatever hang-ups or worries I had about directing. I can't give this to someone else because if they mess it up, it would be really painful; if I mess it up, I can live with it because at least I've tried. I was too much of a scaredy-cat, as my kids would say."

Forbes' determination to tell her own story stems from her wish "to write about what it's like when you love somebody who's having a really hard time and struggling with something, like mental illness," she says. "I feel like in movies, too often, those people aren't represented the way they are in life."

In the offbeat family drama, Forbes' father is played by Mark Ruffalo and her mother by Zoe Saldana. Intriguingly, her younger self is played by the then 11-year-old Imogene Wolodarsky (now 13), the writer-director's real-life middle daughter with her husband, writer-producer Wally Wolodarsky. Even their eldest daughter made a cameo appearance in a lacrosse scene.

Both Ruffalo and Saldana were attached to the project early after they read the script and liked it. "The nice thing about doing a movie like this is that nobody does it for the money - there was no money," says Forbes with a laugh. Although she had to work around the schedules of both Hollywood stars, she's glad to have found her ideal actors for the indie production.

"They were very warm and parental towards the kids," she says. "A lot of the rehearsals were just getting used to being a family. And I always knew that, in the end, it was a story about two people who do love each other but cannot live together. It's very important that the chemistry is there."

Although Forbes' compassionate tale is only reaching a wider audience belatedly (its American cinema release was on June 19), it seems like a much longer time coming for the filmmaker to see her life story reconstructed on screen. "I feel like I started writing it 30 years ago," she sighs. "I've been trying to figure out how to tell this story. It took me a very long time wrestling with it."

Forbes had been a comedy writer for most of her professional career before Infinitely Polar Bear. After working for the humour magazine The Harvard Lampoon while a Harvard undergraduate, she went on to write for The Larry Sanders Show before turning to screenwriting, with credits such as The Rocker (2008), Monsters vs. Aliens (2009) and Diary of a Wimpy Kid: Dog Days (2012).

"I like writing comedy, but I wouldn't say that I'm a heavy jokes person. I mean, I write good jokes, but my strength is more character-based stuff. I do always like the emotions," says Forbes, who sees an unusual parallel between her approach and that of Russian literature.

"I like Russian novelists because I feel like they enjoy the highs and the lows. They seem to write with a lot of energy about both happy things and sad things. I've always really liked that about [Fyodor] Dostoyevsky; [these writers] can be ecstatic about miserable things. And that's what family is about: you go through sad things together."

Forbes cites Noah Baumbach's The Squid and the Whale (2005), Francois Truffaut's The 400 Blows (1959) and James L. Brooks' Terms of Endearment (1983) as her bittersweet movie inspirations. "I see things through a funny prism," she says. "I didn't want [my film] to be heavy. I wanted it to be buoyant - a celebration of the good and even the bad."

Forbes is looking forward to forming a writing-directing team with her husband as they further explore the market for family comedies. Meanwhile, after making her big-screen debut and hosting Q&A sessions alongside her mother at various major festivals such as Sundance and Toronto, Imogene Wolodarsky has already taken her next step as a budding actress.

Last November, she played a small part - as one of the three little pigs - in her school's production of the Stephen Sondheim musical Into the Woods.

"She's happy with her part," says Forbes of her daughter. "I don't know, she wants to act some more. She's funny, she's so cute. My husband and I say, 'If you wanna act, work on your writing and don't be afraid of directing. You can do all things, like Lena Dunham. Do it all!'"

Infinitely Polar Bear opens on June 25