When actor Tim Daly introduces himself he is often met with “Like Tyne Daly?” To which he responds, “Actually yes. She’s my sister.”

Ten years apart, siblings Tim and Tyne Daly are more dissimilar than not. Tyne is the elder and emanates quiet strength and wisdom; Tim relaxes immediately, telling stories, sharing opinions, musing aloud. The pair make for ideal scene partners, yet they’ve never appeared onstage together—until now.

The brother-sister duo open the 40th anniversary season of the Dorset Theatre Festival in Dorset, Vermont June 22, with a play penned for them by playwright Theresa Rebeck (Smash), directed by Adrienne Campbell-Holt. After working with Tim a few years ago, the author wanted to collaborate again. “I said, ‘Why don’t you write a play for Tyne and me?’” Tim recalls. “And she said, ‘Don’t say that unless you mean it.’” A year later, Rebeck handed him Downstairs. Tyne was intrigued when she first read the script, excited by the “sneaky, funny terror in a way that I find very entertaining.”

Tim plays the struggling Teddy, who moves into his sister Irene’s basement (much to her husband’s chagrin) after hitting an all-time low. Digging into a past laden with betrayal and disappointment, who is saving whom becomes murky as the siblings’ story plays out.

“The fact that they don’t line up at all is perfect,” says Tyne of the two characters. “[It’s] true because everybody has different parents. Even if they have the same biological parents, they have different parents.”

“What is more rich and fertile and fecund territory than families?” asks Tim. “So here we are.” Though the blood relatives have appeared on the same screen, they are still figuring out just how to work together in theatre. “We’ve already revealed too much about our past to our fellow castmates,” Tim admits.

Aside from working with his sister onstage for the first time, the play provides him another rare opportunity. Typically cast as an upstanding, polished do-gooder—like Dr. Pete Wilder (ABC’s Private Practice) or husband to the Secretary of State (CBS’ Madam Secretary)—Tim now sinks his teeth into a messier character. “[Teddy] goes all the way down to literally the gutter, and Theresa likes me in the gutter,” he says with a laugh. “I think Theresa sees, somehow, into some of the damage we all have and she likes to exploit it.”

Though Tim hesitated about going into the family business and becoming an actor, Tyne—a Tony Award and six-time Emmy winner—sums up her chosen career succinctly. “I knew from a tender age,” she says.

Perhaps because he is related to her, Tim can pinpoint his sister’s ineffable qualities. “Tyne is so amazingly adept at knowing how to make moments work for an audience and how long to hold things and how to keep people excited and…uncomfortable, but also surprised. That’s an incredible kind of technique that she has that I will always like to learn from,” he confesses. “But there’s a ferocity to Tyne’s work and it’s so compelling. It’s like you can’t take your eyes off her because she has the kind of energy that is riveting.”

Tyne brings that same energy to describing her admiration for her brother’s talents. “His choices. His depth,” Tyne says. “I like watching him be the leader. The category called ‘Leading Man’ or ‘Leading Woman’ has nothing to do with ‘star’ or ‘brand’ or any of those things that they’re throwing around. He’s a wonderful leading man.”

While Tim has led on television more than stage, there is no question about the Dalys’ reverence for theatre. “Growing up, theatre was our temple,” says Tim. “It was a holy place. You were humbled before this amazing thing.”

“It wasn’t about you,” Tyne adds. “You were supposed to be in service of it. It was not supposed to be in service of you.

“The life lesson from my parents for me was that acting was not only a noble profession, but a useful profession that people actually couldn’t do without,” she says. “What you were up to was something to be honored.

“I do think our profession is necessary,” she continues. “And it sustains me to think that there’s a whole lot of other people in the history of human beings that have suffered before and been nervous before and been fierce about it before and that makes me feel comforted.”