Canadian actor-director Sarah Polley has revealed her new film Stories We Tell is inspired by a painful personal incident in which she learned the man who is her father was not her biological father.

Polley’s documentary film, produced by the National Film Board, made its debut at the Venice Film Festival on Thursday.

But Polley said she would not give any interviews on the festival circuit, preferring to tell the story behind the film in a blog on the NFB website.

In the blog, Polley says a journalist discovered the secret of her parentage in 2007, but she was able to convince him or her to keep the story quiet so she could make a film that examined the issue herself.

sarah-polley-250-02806671

Sarah Polley says it was a painful journey to delve into her own conception in Stories We Tell. (Canadian Press)

"Making this film was the hardest thing I’ve ever done. It took five years and tormented me. I didn’t want to make it, and I wanted to give up many times along the way, but I also didn’t want this story to be out there in the words of someone other than the many people who lived it," Polley writes.

Polley, whose previous films include Take This Waltz and Away from Her, lost her mother at age 11, but reveals that she met her biological father by accident and that the biological connection was confirmed by a DNA test.

She calls her father’s response to news of her mother's affair "extraordinary."

"His chief concern, almost immediately, was that my siblings and I not put any blame on my mother for her straying outside of their marriage. He was candid about his own lack of responsiveness towards her and how that may have led her to the point where she sought out the affection of another person. And then he began to write. And write and write and write," Polley says in the blog.

This was part of the inspiration for Stories We Tell, along with the story as told by her biological father and by others who could speak about it.

Polley said making Stories We Tell allowed her to "be a detective in my own life" and learn more about her mother.

"I’m not claiming that my film lacks self involvement but what I wanted most was to examine the many versions of this story, how people held onto them, how they agreed and disagreed with each other, and how powerful and necessary creating narrative is for us to make sense of our bewildering lives. I wanted the story told in the words of everyone I could find who could speak about it," she said.

The Venice Film Festival opens Thursday with The Reluctant Fundamentalist, a film by Mira Nair based on the award-winning book. Polley’s film is not among the 18 movies competing for the festival’s top prize.