About an hour into his dramatic portrayal of 19th-century American writer Henry David Thoreau, the actor will take a pause, roll up his sleeves and finally crack a smile.

Thoreau has “left the stage,” and Kevin Radaker has come back.

“He rarely smiled,” says Radaker, an English professor at Anderson University and one of the country’s most sought-after historical re-enactors of Thoreau, who is best known for the book “Walden” and his essay “Civil Disobedience.”

On Friday, 150 years to the day after Thoreau made a plea for the life of anti-slavery zealot John Brown — who was to be executed after a failed raid at the Harpers Ferry armory — Radaker will stand in the same place, bringing the speech back to life.

In a performance at the same podium Thoreau used in Worcester, Mass., Radaker will stand in the heartland of American history to deliver excerpts of “A Plea for Captain John Brown.” He also will perform in Boston and Concord, Mass.

“Thoreau took a fervent, fiery stand against the moral darkness of the United States,” said Radaker, a Noblesville resident. “In this speech, he justifies and celebrates the violence (Brown used to fight slavery).”

Radaker embraces the chance to use such events to challenge students — and his audience — to think twice before condemning those who break laws while fighting for justice.

“His performance really enhanced my perception of Thoreau as a risk-taking, passionate thinker who wasn’t afraid to break some rules,” said Anderson University student Heike Baird, 21, a senior English major. “Thoreau wasn’t just some bizarre guy who liked to hang out alone in the forest; there was a whole societal commentary behind it.”

Radaker, 53, remembers discovering that himself when he opened Thoreau’s “Walden” in high school and felt an instant connection.

“I was drawn by his message of marching to the beat of a different drummer,” Radaker said. “His voice helped me have the courage to not always conform to my friends, which was huge in high school.”

Radaker refers to himself as a Thoreau scholar, not because he’s written a book (he hasn’t, at least not yet) but because of the enormous research he does. Before performances, he rereads stacks of note cards with tidbits about Thoreau’s life and works. He memorizes passages. He works on his old English accent and onstage demeanor.

“I’ve merely found a different way to educate,” he said.

Inspired by a high school teacher who would often slip into the role of a historical character in class, Radaker sees himself as another Hal Holbrook, who famously portrayed Mark Twain. After a brief role as Thoreau in a stage play in 1977, Radaker was invited in the early 1990s to join the High Plains Chautauqua, a traveling troupe of historical characters who played for crowds across the nation.

Since then, he has honed his performance and developed better ways to glue on the famous Thoreau whiskers so they don’t slip off on a hot night. He’s played the role more than 300 times in California, New Hampshire and many states in between.

Michael Frederick, executive director of the Concord-based Thoreau Society, sought out Radaker to help anchor this coming weekend’s 150th anniversary of the John Brown affair.

“He has come to us highly, highly recommended,” Frederick said. “He is really well-known for the quality of his work.”

Sometimes that quality slips into his normal speech, especially while he is teaching.

“He has taken so much time to perfect his New England accent that I think it slips into his speech pretty consistently,” said Baird, who has been in four of Radaker’s American history classes. “He definitely sounds more like a Bostonite than someone living in the Midwest.”

Radaker, in his own more humble voice, said this weekend’s performances will be special.

“This is really one of the most prestigious events, and to be asked is very much an honor for me.”