Sarah Polley

"Stories We Tell" director Sarah Polley. (E. Jason Wambsgans/Chicago Tribune / May 15, 2013)

All families have their mysteries, their complications. Some more overt than others.

For the most part, though, we go through life not really questioning the certainties. These are our parents. These are our siblings. These are the stories we tell.

What happens when the narrative shifts beneath your feet?

That's the underlying theme of the absorbing new documentary "Stories We Tell" from Canadian actress Sarah Polley, who learns as an adult that she is the product of her mother's extramarital affair."It felt a little bit too melodramatic to be real," Polley said last month on a tour for the film. The film centers on this discovery and how her father, siblings and her mother's old friends and cohorts respond to the news and then filter it through their own recollections of their lives together.

"Who cares about our stupid family?" says one of her sisters at the film's outset, a small, wry smile on her face. You sense that Polley, 34, has the same thought. What she's really after is something beyond her own back story, something more philosophical: Can we trust our own memories?

"I didn't want to make a personal essay documentary about my life," she said. "And I really didn't want to make a film as a kind of therapy. I think what was interesting to me was all the competing narratives and the way we all have this need — this essentially human need — to make narratives out of our lives. We were all telling the story in such different ways."

Polley's last major role as an actress was as John Adams' daughter on the HBO miniseries starring Paul Giamatti. Gradually, over the past six years or so, her focus has shifted to writing and directing, including the 2006 indie "Away From Her" (starring Julie Christie) and 2011's "Take This Waltz" (with Michelle Williams and Seth Rogen).

"Stories We Tell" is her first documentary, fully financed by the National Film Board of Canada. "If I could do it that way again, I would make documentaries for the rest of my life," she said.

Old family movies shot on Super 8 are threaded throughout, depicting a high-spirited home life that is dominated — enchantingly so — by the family's matriarch, Diane Polley, a casting director and occasional theater actor who died when Polley was 11.

Michael, her father, never remarried and raised Polley and her two older siblings in Toronto. There were two more siblings as well, from her mother's previous marriage. Polley is the baby of the family by a number of years, and her brothers and sisters would joke that her real father was someone other than Michael. No one can explain how or why this idea came about — or why it eventually took hold as more than a joke.

Polley was 27 when she found out definitively that her biological father was a man with whom her mother had an affair while performing in play in Montreal. His name is Harry, a wild-haired film producer who appears thrilled to have Polley in his life, but has strong feelings about who precisely should have ownership of this story. (Hint: Not Polley)

"He did participate for three or four days in a film that he had questions about," said Polley, "so I think that was really magnanimous of him."

Among Polley's family members, the revelation is an object of quiet curiosity . Memories don't always sync up. (One of the film's more sly techniques — here be spoilers — is to include old Super 8 footage of the family that is later revealed to be a re-creation, highlighting just how malleable memories can be.)

Polley, the rarely seen linchpin, mostly keeps the camera pointed away from herself. Nor do we learn much about her own thought process.

"It wasn't protecting my privacy, because everything in my life is out there anyway," she said. "I just didn't know why I was making this film, or why I would ever want to show it to people.

"I never really felt committed to the sense that this was a smart thing to be doing — or this would not be harmful to me in some way, or to other people in some way. So I think I never felt really at peace about any of it until the film came out and nobody was upset."

Despite the sleepless nights and revealing subject matter, the project, Polley said, was easier to get off the ground than her last two narrative features, where "you're constantly dealing with people who are looking at the bottom line, so it's about, how can we make this more accessible, more marketable, more box office?

"There were so many conversations about my first film," she said, "where it was like, OK, but is it a rom-com? Is it a drama? Is it a love story? And if you chose one thing, they'll find a bunch of reasons why it's not that thing. I think the idea of making films to fit genres is really weird. It's like it's devised by people who don't make films. It's a way of talking about it, as opposed to actually making a story that has meaning.

"So the process of making a documentary was great, but that's also because I'm in Canada and there's public financing for documentaries, which makes a big difference."

These days Polley is back in the world of narrative features, working on an adaptation of Margaret Atwood's historical novel "Alias Grace," about a notorious mid-19th century murder in upper Canada.

"I love it," she said of the process — which has been the very inverse of her hunt for the truth in her documentary.

"As much as you may doubt yourself, you don't doubt the story."

nmetz@tribune.com