Mark Ruffalo is a master at playing a certain type of earnest character who often wears a quizzical expression — not because he’s slow on the uptake, but because he’s the smartest person in the room and he has questions no one else has even thought to ask.

As a crusading attorney in “Dark Waters,” Ruffalo has a role squarely in his comfort zone, and it’s no surprise he knocks if out of the park in a true-life David v. Corporate Goliath legal thriller with echoes of films such as “A Civil Action” and “Erin Brockovich.”

Ruffalo’s Robert Bilott isn’t the kind of guy who’s going to punch out the villain or give one of those grandiose, theatrical courtroom speeches we see in a John Grisham movie.

He’s a noble pest. He’ll attend a fancy fundraiser just so he can corner the powerful executive who’s been dodging his calls. He’ll quietly but firmly stand up to the partners at his law firm who want him to drop a controversial case. He’ll knock on doors, he’ll make cold calls, he’ll forgo sleep, he’ll rattle cages, he’ll outwork you and he’ll outlast you.

Directed in a relatively straightforward style by the usually quirky, indie-leaning Todd Haynes (“Safe,” “I’m Not There,” “Carol”), “Dark Waters” isn’t as flashy or as shamelessly audience-pleasing as “Erin Brockovich.”

Robert is every bit as determined as any underdog attorney in recent movie history.

Over the course of a dozen years, he puts his career in jeopardy, he works so hard he literally has a stroke, and he almost loses his family as he refuses to surrender in a seemingly unwinnable battle against the mighty and deep-pocketed DuPont company.

It all starts with one Wilbur Tennant, a West Virginia farmer who is convinced DuPont has poisoned his cattle, his land — and maybe his family, as well.

The great character actor Bill Camp plays Wilbur, who speaks with such a heavy-accented growl we almost need subtitles to understand him. Wilbur stomps into the offices of the Cincinnati law firm where Robert works, hoping Robert will represent him because Wilbur is acquainted with Robert’s grandmother, who lives in the same depressed, working-class West Virginia town where Robert grew up.

Robert explains he can’t take the case. Yes, his firm specializes in environmental law, but they’re all about DEFENDING the big-time corporations.

“Dark Waters” is based on the 2016 New York Times Magazine article “The Lawyer Who Became DuPont’s Worst Nightmare,” which chronicles a case that stretched on more than a decade.

Director Haynes is almost too faithful to the timeline, resulting in some lulls in momentum, especially in the middle third of the story.

Of course, we know Robert will win the day. They don’t make movies about crusading attorneys who get crushed in court by corporate monoliths.

“Dark Waters”

127 minutes

Rating: PG-13 for thematic content, some disturbing images and strong language

3 stars