If Blake Lively were sitting in front of me and describing her latest role as “my favorite,” I might be wont to dismiss it as a flippant remark, something like a friend offhandedly reassuring you that she just loves her new shoes. But Romi, the unapologetic, radical-minded, free-loving freegan of Eve Ensler’s new play “O.P.C.” isn’t tackled by the Gossip Girl. Romi is the latest project for the cerebral Olivia Thirlby, and when she says Romi is her favorite, and “O.P.C.,” “as a piece of dramatic material” is her “favorite one,” well, she means it.

“The whole psychology of my character is something I find very appealing and compelling,” says Thirlby, her gaze intense, her words careful and deliberate, her hands warming themselves on a reusable cup that harbors a bobbing tea bag. “[Romi] is someone who has ascended out of the system that we are so hopelessly stuck in and has carved out a life for herself that she believes is authentic and that makes her feel good. I really respect that. It takes such fearlessness.”

For Thirlby, a Shakespearean-trained stage and screen actor who is known for Juno and Dredd—she was included in Vanity Fair’s Hollywood’s New Wave roundup in 2008 following the former, alongside Lively and contemporaries such as Emma Stone, Kristen Stewart, and Amanda Seyfried—this character is the one she’s been looking for. “Oftentimes when you read a great role,” she says, “you never get the opportunity to play it, so this is exciting for me.”

Part of her excitement stems out of working with playwright and activist Eve Ensler, who could be found on set throughout rehearsal. Though she was only 10 when Ensler premiered “The Vagina Monologues” in 1996, Thirlby fondly recalls participating in a V-Day event, the play’s corresponding nonprofit movement, which has raised over $75 million for women’s anti-violence groups, as a preteen. For “O.P.C.,” directed by Pesha Rudnick, currently in previews at A.R.T.’s Loeb Drama Center, opening tonight, Ensler unsurprisingly packs in the politics. With Romi, a dumpster-diving freegan fashionista who imagines dresses out of apricot skins, and her mother Smith (played by Kate Mulligan), a mainstream liberal running for Senate, comedic intergenerational differences and debate act as a microcosm with which Ensler speaks to society’s ills, from income inequality to global warming to government surveillance.

“Yes, it’s absolutely cyclical that every generation thinks that it’s really bad and the world is going to end,” says Thirby, of the show’s message—a quasi-call to action, even. “But then it doesn’t, and then there is another generation who gets to think that. Except we are in the midst of a global climate crisis that’s not going away. And that’s not a question. It is not going away. We won’t have another generation to have this cycle with if we don’t address what’s happening right now.”

For Romi, her artwork—patchworked vibrant garments made from recycled materials (which the A.R.T. costuming team had the challenge of making from second-hand goods like yoga mats and lawn chairs)—doubles as a form of activism. Thirlby admires this quality in her character and aspires to it herself.

“There are so many things we have to do when we participate in this society that forces us to de-integrate ourselves. We have work, then we have life. Family or friends. Our art or our job. I think this kind of compartmentalizing is actually the source of a lot of unhappiness. Romi’s genius is that she’s figured out a way to integrate all parts of her being so that she doesn’t have to choose, so that she doesn’t have to wear different hats. She’s taken all the different hats, sews them together, and wears them as a dress.

“As a person, I struggled for a long time in how to integrate my job as an actress with the art of acting, which may sound strange but they actually don’t always go together,” says Thirlby. She juxtaposes the art form of theater and film with what she sees as the polarizing “Hollywood” component. Thoughtfully, slowly, she says, “I very much want to just be.”

“I don’t want there to be work and life. I want life and work to be one giant symphony. The amazing part of this job,” she says referring specifically to this production, “is that it makes me feel like I can integrate all parts of myself. Roles where you can practice this don’t exist. This is an anomaly, this role. So it is deeply, deeply special to me.”

O.P.C. LOEB DRAMA CENTER. NOW IN PREVIEWS, OPENING WED 12.3, RUNS THROUGH 1.5. SHOWTIMES AND TICKET PRICES, VISIT AMERICANREPERTORYTHEATER.ORG