Under the harsh florescent lights of the rehearsal room, Roger Grunwald transforms. With only a twist of his shoulder, he changes from a stern Ukrainian man in a subway car into Polish Holocaust survivor Schmuel Berkowicz. Then he stops and does it again.

“Maybe he [needs to be] a little more upright,” director Nancy Carlin instructs Grunwald.

The actor and director are getting ready for the opening of Grunwald’s one-man show, “The Obligation,” which will run Oct. 12 through Nov. 5 at Potrero Stage in San Francisco. In it, Grunwald portrays not only the central figure of the piece, Schmuel Berkowicz of Bialystok, but also the many other figures — from an SS officer to a naive American nurse — that come to life as Schmuel recounts his history.

“He is speaking to the audience in the play as if he were a survivor speaking to students at City College of New York,” Grunwald notes.

The result confronts deep truths about identity and othering through stories featuring both laughter and pain, all brought alive by Grunwald — himself the child of a survivor.

“He’s a wonderful performer, storyteller and writer,” Carlin says.

The play is the continuation of an earlier project by Grunwald called the Mitzvah Project; it combined a short play (“The Mitzvah”) with a history lesson and Q&A session. The text and characters from “The Mitzvah,” co-authored by Grunwald and Annie McGreevey, are encapsulated in “The Obligation.”

Like the earlier project, the new play also will be followed by conversations with guest speakers. The lineup includes the Very Rev. Dr. Alan Jones (Grace Cathedral in San Francisco) on Oct. 20; author Elizabeth Rosner, the daughter of Holocaust survivors (“Survivor Café: The Legacy of Trauma and the Labyrinth of Memory”) on Oct. 21; Morgan Blum Schneider (the Holocaust Center at S.F.-based Jewish Family and Children’s Services) on Oct. 22; and Rabbi Peretz Wolf-Prusan (Lehrhaus Judaica) on Oct. 29.

Grunwald has been touring with the Mitzvah Project since early 2015 — taking it to schools and synagogues, but also some unexpected places.

“Some of the most powerful examples are when I did it in Catholic high schools,” he says.

Telling the story of the Holocaust to people who understood it very little was moving and inspiring, but eventually Grunwald felt he had to create something full-length. After two years of “writing and writing and writing,” he says, he had “The Obligation,” which tackles the same kinds of themes but on a larger scale.

“Do we have the capacity as a species to transform, to let go of labeling ourselves and others?” Grunwald posits.

Grunwald, who grew up in San Francisco and went to UC Berkeley, has spent the past 40 years living and doing theater in New York City. He is one of the early members of the All Stars Project, a nonprofit that uses performance to reach and support young people.

His ancestors were German Jews who, after fleeing to Amsterdam, were eventually sent to camps. “They tried to secure visas to the United States but were unsuccessful,” he says.

Not everyone perished, however. His mother, Lotte, for example, survived the death march from Auschwitz in 1945, and, along with her sister, finally made it to the United States. Grunwald’s aunt, Annie Bodenheimer, is alive at 103. But Grunwald is keenly aware of how fragile the stories of his family — and others — are. With the generation Holocaust survivors now passing away, Grunwald feels a duty to continue connecting people to that legacy through the medium he knows best, the theater.

“The people who were the eyewitnesses to this history are dying,” he says.

Through “The Obligation,” he hopes to use theater’s capacity to touch people in order to remind the audience that the lessons of the Holocaust are always with us.

Or, as Schmuel Berkowicz says at the end of the play: “It’s not just stopping Nazis. We have to stop making a demon, a devil, out of the other.”

“The Obligation” runs Oct. 12–Nov. 5 at Potrero Stage, 1695 18th St., S.F. $16-$56 ($13.50-$41 for Oct. 12-13 previews). potrerostage.org/theobligation