Working with home movies, archival footage, additional film elements, and interviews with her siblings and her father, Michael - an English-born actor whose wise, reflective narration is at the heart of Stories We Tell - Polley pieces together a history, and a mystery, with many a discovery along the way.
She had more than 200 hours of footage to cull and an original 100-page treatment ("single-spaced!" she says) that was both rigorously followed and defiantly abandoned.
Conducting the interviews with her brothers and sisters, with friends of Diane, and with the actors and artists her mother had worked with, often proved tough for Polley to handle. A director with a pair of much-praised and powerful features to her credit - 2006's Away from Her, nominated for two Academy Awards (including best actress for Julie Christie), and 2011's Take This Waltz, with Michelle Williams - Polley found herself buffeted by the real-life revelations. The different versions of her mother's life, revealed through these many different accounts, played a bit like Rashomon.
And then there were Polley's own memories, which meshed, and didn't mesh, with those of her family's.
"There was a scene that we shot, which we didn't include in the film, where we actually shot from my perspective the moment my mother died," she recalls, hunkered over a back booth in a Broadway hotel cafe. "I wasn't in the room, but my journey down the hallway and up the stairs and into the room, and then my aunt stopping me - we shot all that.
"And I remember at some point looking at Iris Ng, who is my cinematographer and who is also a really good friend, and saying, 'I don't think I can be here for this.'
"I remember going and hiding in the kitchen. 'Just tell me when it's done.' "
There's a remarkable clip in Stories We Tell, a black-and-white audition tape of Diane singing "Ain't Misbehavin' " - a tryout to host a big Canadian television show.
Polley had heard all about this audition when she was a little girl - it was down to Diane and one other actress. But it wasn't until Polley started rummaging through the Canadian Broadcasting Corp.'s archives that she discovered that the performance had actually been filmed, and had been saved.
"It was so unbelievable to see what was a very pivotal moment in my mother's life that we talked about all the time, to actually get to see that moment on film," Polley says. "Most people know of pivotal moments in their parents' lives, but you don't have film of it.
"And to get to see on her face - I feel like you can actually see the moments where she realizes she's not going to get it, or how much it means to her, or the fact that she's maybe blowing it. . . .
"It just kills me. Especially having been in that position myself, of being an actor auditioning. It's such a vulnerable, awful spot to be in. And that feeling when you know it's just not going well, and there's nothing you can do to get it back.
"That's so painful. And to have to watch your mother go through that, it was really sad."
Polley, who is 34, married with a baby girl, and living in Toronto, discovered something else during the long, complicated journey she takes in Stories We Tell: that at the core of her two fiction films were couples grappling with the same issues of love and desire, fidelity and fear, truth and lies that her mother and father faced.
"I was unconscious, when making Away From Her and Take This Waltz, of where in me lived this fascination for this kind of subject matter," she says. "Like looking for the truth of what's happening or not happening, and, in both these cases, it being about a woman who's restless and a man who is, in certain ways, pining for her as she's gone somewhere else.
"There are themes that are clearly coming from these archetypes of my parents, but I was totally unaware of that when I was making the films!"
"So now that I've made Stories We Tell, it's strange, because I realized I have been mining the same territory," she says. "With this film, I've gone into the cave and made a film about the actual figures, whereas before, the films were about the shadows of those figures.
"And so now that I've done that, I feel both terrified and liberated. . . . Clearly, this was what was compelling me to make films, to say things. So now that I've made the film that it's actually all about - well, now what?"
Contact Steven Rea at 215-854-5629 or firstname.lastname@example.org, or follow on Twitter @Steven_Rea. Read his blog, "On Movies Online," at www.inquirer.com/onmovies.