"I literally feel like I’m going to wake up and feel like I had the most narcissistic dream of all time,” laughed Sarah Polley with a self-deprecating eye-roll. The 28-year-old actress was speaking about the effusive, heady reception her directorial debut, Away From Her, has already received, having garnered ringing endorsements from critics and audiences well before its May 4 opening date. “There’s a natural life of a Canadian film, which is that it plays for a week in this one theater in downtown Toronto and no one ever hears about it,” she said. “I made this film completely comfortable with that fate.”

In person, Ms. Polley is startlingly small. Not small in the typical Hollywood wow-I-can’t-believe-how midgety-that-big-star-with-all-the-teeth-is, but naturally fine-boned and delicate, with milkweed coloring that seems more suited to a Vermeer painting than a movie star turned auteur. Sitting in an office above Times Square last week, Ms. Polley was clad casually in a brown, long-sleeved scoop-necked shirt, jeans and candy-apple-red shoes. At first glance, she looks a lot younger than her 28 years—and if you passed her on the street, she could almost (almost) be any noticeably pretty and fresh-faced girl visiting town. But the illusion is shattered once she starts to speak. Her thoughts—clearly considered—tumble out in fast, articulate bundles. And she has the steady gaze and tell-tale habit of stopping and waiting for the next question when she’s finished answering the last that speaks to a lifetime spent in the public eye.

Ms. Polley was born into a showbiz family in Toronto, to a stage-actor father and casting-director mother. She got into the business at the age of 6, landing her first major role a few years later as Sally Salt in the Terry Gilliam film The Adventures of Baron Munchausen (not always the most positive experience, she’s since intimated) and finding stardom in her own country with the TV series Road to Avonlea.

“I think it was my idea, originally,” she said of acting, admitting that by the age of 8 she probably would have been happier to be back in school, but also quick to assure that her mother and father weren’t typically pushy stage parents. “But what happens is that you get so much pride from your parents for accomplishing that kind of thing that it becomes a pressure all in itself. For me, it would have been hard for me to let go of all that attention.”

Slideshow: "The Cutest Auteur"As True Hollywood Stories has helpfully cataloged for us, and as Ms. Polley described it, the road of child stars is a notoriously fraught one. “It’s a toxic combination for a child actor, one of pampering and neglect which leads to a kind of absolute insecurity and absolute egotism,” Ms. Polley said. “It’s an unfathomably difficult thing to unravel. Even it its subtlest form, the idea that you’re forming your own identity as a child while you’re in this job where you’re being congratulated for pretending to be other people is really bizarre.”

One consistency in the characters that Ms. Polley has portrayed is their tendency to be almost heartbreakingly wise, with a weary-before-their-time pathos brought on by the absence of childhood naiveté. Was it a quality she was aware of?

“I think as a childhood actor you acquire a kind of precociousness,” she answered carefully. “But I don’t know how deep it goes. Do you know what I mean? I think most people I’ve met that have grown up as child actors have been able to convey a sense of being older than they are, but it doesn’t necessarily go a lot deeper than having learned that behavior, and”—she laughed—“in fact, it masks a very deep immaturity.” She has no regrets, but she is aware that “it was kind of luck-of-the-draw that it didn’t turn out badly for me. There were a lot of moments when it really could have.”

Ms. Polley’s mother passed away from cancer when she was 11, yet she continued to work with characteristic intensity, all the while becoming more politically active, toiling alongside like-minded lefties to oust Mike Harris, the Conservative premier of Ontario. In 1997, she starred in The Sweet Hereafter, a quietly devastating film that won the Grand Jury Prize at Cannes and nabbed two Oscar nominations for screenwriter/director Atom Egoyan (an executive producer on Away From Her and “a huge influence,” Ms. Polley said). Her performance as the lone, wheelchair-bound survivor from a school-bus accident drew raves.

Hollywood took notice. The blockbuster-shy actress continued to stay within the independent-film realm, but after solid performances in the late 90’s in films like Guinevere, David Cronenberg’s eXistenZ and Doug Liman’s Go, she found herself at a crossroads when she was cast in the part of Penny Lane in Cameron Crowe’s Almost Famous. Two months into rehearsals, she quit the production. Kate Hudson replaced her to great acclaim, and the rest is Us Weekly history. “I just found myself in a situation where I was playing this part I didn’t think I was right for,” she said. “Cameron felt like I was right for it, but I couldn’t quite see it. There was just this sort of machine around it, you know? You felt like whoever was going to play this part was going to have, like, this giant life, and I didn’t want a giant life. I wanted a pretty normal life. It was a kind of survival for me, because I actually don’t know if I would have survived that life.”

Around that same time, Ms. Polley became interested in writing and directing; she made her first film short when she was 20. “I made this one thing on a whim and discovered by accident that I loved it more than anything I had ever done,” she said. When she was 21, after completing the Hal Hartley film No Such Thing (co-starring Julie Christie), she picked up The New Yorker on the flight home from Iceland and read Alice Munro’s short story, “The Bear Comes Over the Mountain.” “It wouldn’t stop growing in my head as a film. It felt like such an urgent thing in my life to make it. And I couldn’t stop imagining Julie Christie in that part,” she said. Years ticked by, but Ms. Polley continued to think about it. “I thought there was no way I was going to be able to get the rights,” she said. “I thought they’d be taken or too expensive—something. Then all of a sudden we had the rights, and I thought: ‘Oh my God, I have to make this into a film!’

“It was nerve-wracking and thrilling,” she said about stepping onto the set for the first time. “But you know, this was a story that I absolutely loved and, in a strange way, making the film was just an excuse to walk around inside it for a few years. It’s such an amazing thing to fall in love with this fictional world and then get a chance to inhabit it.”

Away From Her, adapted for the screen by Ms. Polley, is about Fiona (Ms. Christie) and Grant (Gordon Pinsent), a couple who have been married for 44 years when Fiona starts showing symptoms of Alzheimer’s. When she voluntarily chooses to be put in an assisted-living establishment, Fiona ends up developing a strong attachment to a fellow patient and no longer recognizing her mate, much to the confusion of her dogged husband. The disease dredges up issues long buried from the couple’s past, and the end result is terrifically poignant. It’s a grown-up film that examines what fidelity, devotion and love look like from the end of the journey rather than the start. “I was so interested in the fact that it was told from a male point of view, the coming to terms with who they’ve been and actually becoming capable of something that they’ve never been capable of in their life when it’s almost too late,” Ms. Polley said. “It was gorgeous to think about unconditional love in that way.”

Julie Christie, as beautiful on-screen in her mid-60’s playing Fiona as she was in the 60’s (“When I first met with Julie about it, I was looking at her perfect legs in the mini-skirt she was wearing that day and her perfect hands, and I was thinking, ‘This isn’t going to work—no one will ever believe me that she’s someone in a retirement home!’”) is pitch-perfect (the inevitable Oscar chatter has already begun). “I was in love with her in Darling and Doctor Zhivago, just like everyone else,” said Ms. Polley, though Ms. Christie took some convincing, turning Ms. Polley down repeatedly before agreeing to take the role. “She’s a reluctant actor, and she definitely requires a bit of convincing to get her off her farm and into a movie. It did take a lot of persistence.”

Ms. Polley’s husband of four years, David Wharnsby, served as editor on the film. “It’s sort of a crazy thing, in a way—to edit a movie with your husband, when it’s a movie about marriage. We learned a ton about ourselves and about our relationship. It was great to be in an environment where you can’t walk away from disagreements.” They met when Mr. Wharnsby edited Ms. Polley’s first short. “We had a bizarre coming-together,” she said. “We were sort of friends, and I think in the first two or three weeks I met him, I sort of felt like, We should be in love at this point. I remember him saying that his definition of love is not what happens in the first three weeks. His parents had this 45-year marriage where they’d been through some really hard stuff, but at the end of the day, he’d still see these incredible moments of tenderness between them that were possible even after everything they’d been through. I think it was really getting to know their relationship and seeing them together that made me really captivated. They inspired a lot of what’s in the film.”

Cinematically speaking, love stories tend to be told from their beginnings, during the time of passion and discovery. But that conventional line didn’t interest Ms. Polley. “What happens when real life thrashes you around?” she said. “I think it’s so weird we’re not willing to embrace the most interesting part of a relationship. It’s full of failure and it’s never what you expected. It’s like we need these fairy tales somehow, but it really does us a disservice. It’s far more interesting to go through life and really let each other down. And then what? That’s what’s interesting.”

The process of directing has given her a fresh perspective on acting. “I see it as a really interesting job now,” she said, crediting Ms. Christie and Mr. Pinsent’s work on the film as inspiration. “It made me want to give that much of myself the next time I acted.” She’s due to start work in a week on a mini-series about John Adams for HBO, and plans both to write an original screenplay to direct and to adapt another one. “I’ve never done anything like this before,” she said of her new multitasking lifestyle. “And I’m acting at the same time—so I have no idea what will come of anything.”

Despite this uncertainty, Ms. Polley’s life is just what she wants it to be. She lives with her husband in what she describes as a great neighborhood in Toronto, where they do “nothing exciting.” She picks her projects by whether or not she’d pay to go see them in a theater. “For me, this is a perfect life,” Ms. Polley said. “I get to work with filmmakers I love, be in independent films I’d see in a theater, but my life is completely manageable and not spiraling into something I can’t handle. It’s a perfect balance.”