Ann Dowd addressed her fellow actors from the movie “Mass,” which premiered at the Sundance Film Festival: “Guys, can you imagine if it was a play? We’d be in Bellevue.” 

“Forget it!” said co-star Martha Plimpton.

The veteran actor Fran Kranz wrote and directed “Mass” as his first feature. The movie is about two sets of parents meeting years after a school shooting — one whose son was murdered, the other, the parents of the shooter. The movie is set almost entirely in one room as the four of them — Gail (Plimpton) and Jay (Jason Isaacs), Linda (Dowd) and Richard (Reed Birney — hash it out. As Variety chief critic Owen Gleiberman wrote in his review, “It’s like a slow-burn group confession that’s also a debate, and it invites us to take a journey into the souls of all four of these people. Sitting with them in that room, we travel somewhere.” 

Kranz said he surprised himself when he wrote “Mass,” having had other kinds of ideas for movies for years. But the day of the Parkland shooting, he started thinking about it. “I’ve never worked harder on anything,” he said at Variety’s Sundance Studio, presented by AT&T TV. “I’ve never had such a singular, obsessive focus in my life. It’s strange.”

“I’m not trying to portray myself as an overly empathetic person,” Kranz continued. “But I couldn’t look away — thinking about these people, the lives and the families affected.”  

Because of the intensity of the story, and the demands on the page, Kranz said he wanted theater actors for the main roles. But he said, “I never imagined these guys — I have a star cast here.”

Plimpton said she avoided doing research to play Gail, and “used the script.”

I’m a very matter-of-fact kind of actor,” she said. “I don’t need a whole lot of oogly boogly shit. We were all working together, and going through this process together, and unspooling this thing together.”

Isaacs, who plays her husband, said, “Unlike Martha, I like a lot of oogly boogly shit.” He said the two of them worked together to figure out what happened in their marriage since their son was killed.

“Martha and I did a lot of talking, and even arguing — like Jay and Gail would have done,” Isaacs said. “About what the rules were going to be, and hot words we had to avoid, and the approaches we needed to have.”

“I don’t know why I don’t think that’s not oogly boogly,” Plimpton said. “He’s totally right.”

Birney said opportunities like “Mass” almost never come along. “Even in plays, it’s so rare that we have the opportunities to do something that’s just about the acting,” he said. “Nothing blows up, nothing else happens except the magic that actors can create. And that’s so rare.”

For Dowd’s part, she said that Birney is a “wonderful actor to work with.” She also talked about how much she relied on Kranz. “I felt like a child, actually — so needing to have his words,” she said. “Not just the script, but his physical presence and his awareness. And knowing how to take us through. Because otherwise, man, I don’t know how you do it.”

Dowd got choked up talking about it: “I’ve said this many times, but these now are my beautiful friends and family for my life. Because you don’t go to that place and ever forget it, you know what I’m saying?”