10/12/2012 04:32 EDT | Updated 12/12/2012 05:12 EST

Sarah Polley explores family secrets in doc Stories We Tell

Six years is a long time for a high-profile figure to keep the lid on a secret, but Canadian filmmaker Sarah Polley succeeded and is now raising the curtain on a doc that sheds light on a very personal revelation from inside her own family.

After playing to limited audiences at the Venice and Toronto International Film Festivals, Polley's new documentary Stories We Tell is being released in Canadian theatres.

In the film, the Toronto actor-turned-director digs for the truth surrounding her late mother Diane — who died of cancer when Polley was 11 — via stories and memories shared by her father Michael Polley, her siblings and friends of her mom. The catalyst was discovering, in 2006, that the man who raised her was not her biological father but that she is the product of an affair her mother had in the 1970s.

"I really wanted this story to be told through this film and not through interviews and not through pieces people would write," Polley told CBC News.

"I really wanted this film to make that revelation because it was something so personal to me and I wanted it to be told in the words of the people I love. What was amazing to me was that it actually worked because so many journalists in Canada knew this story for so long. So many people in the media and entertainment [communities] knew it. I was just sure somebody somewhere would talk about it."

The journalists who had learned the news agreed to remain silent about it, including arts writer Matthew Hays, who is friends with Polley's biological father, Montreal producer Harry Gulkin.

"I felt that I couldn't break this story and I didn't really want to," Hays revealed. "Harry had said 'This is a private matter for us. We're still working it out.'"

For the former child star, who grew up on camera in TV's Road to Avonlea and films like The Sweet Hereafter, what was intriguing about the revelation (confirmed through DNA testing) was how the various members of her family began trying to piece together the same story — reanalyzing their memories of her mother — but how each tale was distinct.

"What became really interesting to me and captivating was the way we were so attached to narrative and how desperately we need to create this narrative to make sense of events that were a little bit bewildering to us," she said, admitting that everyone involved had reservations about the personal nature of the exploration.

"I thought of that kind of like a human need: the way we create a story out of something that has happened in our life to make sense of it."

Stories We Tell opens in Toronto Friday, Vancouver and Montreal on Oct. 19 and other Canadian cities this fall.