Whether it’s Sidney Poitier in “To Sir With Love,” Richard Dreyfuss in “Mr. Holland’s Opus,” Michelle Pfeiffer in “Dangerous Minds” or Meryl Streep in “Music of the Heart,” it’s almost inevitable that great actors are eventually cast as great teachers.

But the notion is somewhat new to Viola Davis. “You’re the second person who’s said that to me, and I’d never thought about it,” admits the 47-year-old star, who earned a best actress Oscar nomination for 2011’s “The Help.”

“I think the reason why actors are drawn to that is because teachers are heroic. It’s a chance to play something maybe larger than life.”

In “Won’t Back Down,” which opens today, Davis plays a discouraged teacher who rediscovers her spark for education by joining forces with a working-class mother (Maggie Gyllenhaal) fighting to take over a failing inner-city school.

The film, which is inspired by real events, tackles some of the themes covered by “Waiting for Superman,” the 2010 documentary about the problems of public schools. And it’s already stirring controversy and being slammed by teacher unions, which object to how they’re portrayed in the film.

The issues surrounding the story line are complex, but the dramatic conflict of the movie is fairly clear-cut. In this David and Goliath tale from director- co writer Daniel Barnz, who comes from a family of educators, David is represented by Davis and Gyllenhaal, while Goliath is the inflexible bureaucracy of government and unions.

As someone who’s appeared in several movies that touch on hot-button topics, Davis sees the value of tackling controversial subjects. “When you create stories where that’s the background, the characters will absolutely be interesting. There’s no doubt about it. If you start with that palette, then what will grow out of it is really substantial characters fighting and slaying big dragons, and you always want that as an actor,” she says during a recent phone interview.

Davis, however, isn’t interested in playing heroes without flaws. In this movie, she’s a woman who’s struggling with a divorce and raising a son with learning difficulties as well as coping with career burnout.

“I don’t think it would be easy to come in and say, ‘Hey, let’s start a school.’ I do think that comes at a cost. I think that’s what life is about,” she says of her character. But “the more she stepped toward the challenge, the more she began to come to life. When you do meet a challenge, something that you’re absolutely afraid of, that is really what happens. It brings you back to life.”

Davis, who starred as Aibileen Clark in 2011’s "The Help," a film about black maids working in white households in Jackson, Miss., in the 1960s, was born at the former Singleton Plantation in St. Matthews, and moved to the northeast as a child.

Davis brings empathy and compassion to her portrayals of private, often emotionally guarded women who reveal themselves to be anything but ordinary. She says she’s interested in characters who are going through the motions of life, much like her teacher in “Won’t Back Down,” and in exploring what it would take to wake them up.

“I do believe many of us catapult ourselves to the grave never having tapped in to what’s extraordinary about ourselves,” she muses. “We feel if we’re not a world-class gymnast or a world-class actor or have the greatest body or have a reality TV show that there’s nothing extraordinary about ourselves, which is not the case. I think we take all that potential and we just squelch it. I think it’s interesting in all of these characters, what brings them back to life.”