Fame doesn’t happen to every actor — the kind of fame that gets a face splashed across the magazine racks in the supermarket checkout line and commands multimillion dollar contracts. And not every actor, it turns out, is all that interested.

That’s probably why Sarah Polley was able to sit in the lobby of the Mercer Hotel one recent afternoon without anyone paying her too much mind. She was dressed in street clothes and had done nothing special with her hair. Unlike most performers, who come with a publicist in tow, she was relaxed and unhurried. She began a conversation by trading notes on dermatological issues.

Which is not to say that Ms. Polley is not a celebrity. She just decided to keep it in check. As a child, she became a star on Canadian television; later, she had a career breakthrough in Atom Egoyan’s dark and lyrical 1997 adaptation of Russell Banks’s novel “The Sweet Hereafter.” Her almost alarmingly nuanced performance put her on Hollywood’s radar, and she was subsequently cast as Penny Lane, the teenage groupie in Cameron Crowe’s autobiographical comedy “Almost Famous.”

Then Ms. Polley changed her mind. And now we know every public movement and private affair of her successor, Kate Hudson.

“I think that’s the reason I didn’t do it,” Ms. Polley said. “I really loved Cameron, but it was kind of clear that the part was made to get a lot of attention. I didn’t want my life to go in that direction. It was a relief to me to see how narrowly I escaped that life. I think some people can handle it and remain themselves. But I knew instinctually, if that happened to me then it would destroy me, and I think I was right.”

Now 28, the Toronto native has no regrets, and boasts a string of performances in low-key art-house films, most recently Isabel Coixet’s “The Secret Life of Words” and Wim Wenders’s “Don’t Come Knocking.” More important, she has become a director. “Away From Her,” Ms. Polley’s feature debut, is a surprising and deeply resonant love story about Alzheimer’s disease that is as delicately measured as one of her own performances. The film stars Canadian screen legend Gordon Pinsent and Julie Christie as a 60-something couple whose relationship is radically redefined when her mind begins to slip. Adapted from the Alice Munro story “And the Bear Came Over the Mountain,” which Ms. Polley read on an airplane a few years ago, the film is an uncommon choice for a young, novice director and is shot in an uncommonly stately style. There are plenty of long, stationary shots, abundant vistas of wintry landscapes, and an affection for natural rhythms that allow scenes to play out without many edits. It’s an astonishingly mature piece of work.

“There’s a lot of pressure on people when they are under 35 to do something really cutting edge and really flashy,” Ms. Polley said. “That wasn’t the kind of story I was interested in telling. This story required a lot of grace and a lot of elegance and a lot of simplicity.” If she ever felt as if she might get in over her head, Ms. Polley seemed up to the challenge. “It was intimidating to make a film with these kinds of actors, people I had looked up to for a long time, who were so much older than me. And to make a film about subject matter that is really beyond the range of my own experience. People say write what you know, but I think it’s more important to write what you’re curious about. I was curious about what happens to a relationship over this period of time. It wasn’t a departure for me. I love Bergman movies. I love Terrence Malick, and Kieslowski.”

Despite the heavy tone such influences imply, “Away From Her” will catch audiences off-guard. It’s the sexiest movie about autumnal dementia ever made. Give Ms. Christie the credit for that. The former 1960s “It” girl, who staged her own retreat from the spotlight in the 1980s and ’90s, has seen her own return to the screen parallel Ms. Polley’s path. The pair met when they both acted in Hal Hartley’s “No Such Thing” (2001) and worked together again in “The Secret Life of Words” (2005).

“I really was so captivated by her,” Ms. Polley said, “and so admiring of her curiosity and vividness and generous engagement she has with the world. I find it really intoxicating to be around. She has the sharpest bull—- detector of anyone I ever met.”

The film’s sexual candor may be shocking only because the characters are not 20 years old. It was important for Ms. Polley to mess with this odd taboo. “There’s this weird thing we do, especially in films,” she said. “People lose all sense of sexuality after the age of 50. I thought it was irresponsible to shy away from that.”

That attitude offers a glimpse of Ms. Polley’s creative sensibility: the desire to explore the full dimension of what it means to be human. It’s the highest calling of any art form. It’s not easy, but as she observes, it can get into your bloodstream. She loops back to Mr. Malick and his meditation on World War II, 1998’s “The Thin Red Line.”

“It affected me on that level where you are fundamentally shifted by something you’ve seen,” she said. “It’s like the molecules in your body have moved. It really convinced me of the power of cinema. I actually went from a person with no faith who was in a depression to a person with a little bit of faith who wasn’t depressed anymore.”