What nagged at him was the guilty prospect of letting down his co-star, Treat Williams, who was already a movie star when Mr. Guirgis was in college. A veteran Mamet actor, Mr. Williams now lives year-round not far from the playhouse, on 16 rolling acres with a little orchard and a pond. His excitement about doing the play — a verbally explosive dissection of male power dynamics, set in a Chicago junk shop — had been palpable in the months leading up to production.
“He would always text me or leave voice mails like ‘I can’t wait to do this,’” said Mr. Guirgis, who is 52 to Mr. Williams’s 65. “And sometimes I wouldn’t call him back because I didn’t know what to say.”
If casting Mr. Guirgis was a risk, Mr. Rubin — who first directed him in the late 1990s and “never got over thinking of him as an actor” — says he didn’t believe Mr. Guirgis would cancel on him.
“When he was writing, he was regretting not acting,” Mr. Rubin said. “He procrastinated as a writer, he had trouble as a writer, but he never had trouble as an actor.”
Mr. Guirgis arrived in Dorset only an hour before the “American Buffalo” meet-and-greet, a first-day ritual where the theater staff and visiting artists introduce themselves to one another. He was a day later than originally expected; he had put off traveling as long as possible. That morning in New York, when he caught a train at Penn Station, he took a photograph of himself with his ticket and texted it to the people he’s closest to, to show that he really was going.
“And they needed to see that — the proof,” he said.
In the lobby of the playhouse, he sat down with Mr. Williams — who plays a shady character nicknamed Teach, the Robert Duvall role in the 1977 Broadway premiere — and the third member of the cast, Oliver Palmer, a 23-year-old fresh out of the Tisch School of the Arts at New York University. When it was Mr. Guirgis’s turn to introduce himself to the room, he offered to hold a writing workshop if anyone was interested. “But after I learn the lines,” he added, and got a big laugh.
What was scaring both Mr. Guirgis and Mr. Williams, they said later, was having just 12 days of rehearsals to get the play down before technical rehearsals begin. They have only a single preview before opening, and the run ends Sept. 2. Ambitions for the production beyond Dorset, possibly a tour to regional theaters, might allow them to delve further.
In recent years Mr. Guirgis has had small roles in films, including “Birdman” and Kenneth Lonergan’s “Margaret.” He would have been in Martin McDonagh’s coming “Three Billboards Outside Ebbing, Missouri,” a job he was thrilled to have, but he never got on the plane to go to the shoot. For that, his acting manager fired him.
“And I still haven’t reached out to Martin,” Mr. Guirgis said, attributing his no-show to “depression, anxiety, darkness. Self-destruction.”
Donny, the junk-shop owner in “American Buffalo,” is his first stage role since Brett C. Leonard’s “Guinea Pig Solo” at the Public Theater in 2004. “When I’m acting, it’s the only time that I feel O.K. in my skin, where I feel O.K. about who I am, where I am,” he said. “And if that’s true, which it is, and you don’t do it for years and years and years, it’s going to make you feel worse.”
Similarly, Mr. Guirgis loves being around actors. His career-making early plays, like “Jesus Hopped the ‘A’ Train” and “Our Lady of 121st Street,” were directed by his friend Philip Seymour Hoffman, whose movie career was taking off then, and they were both deeply involved in building Labyrinth. “The Last Days of Judas Iscariot,” also originally directed by Hoffman, put Mr. Guirgis in the news this month when he reluctantly shut down a production at a small San Francisco theater that had cut his text, in violation of copyright.
In 2011 Mr. Guirgis made his Broadway debut with a comedy whose earthy title this newspaper cleans up as “The ____________ With the Hat”; he won a Pulitzer in 2015 for “Between Riverside and Crazy.” Until January, he’d spent three years working with Baz Luhrmann on their sprawling and poetic Netflix series, “The Get Down” — during which period, Mr. Guirgis said, he gained more than 100 pounds and chain-smoked while he wrote.
One night this spring, he said, he woke up at 4 a.m. thinking he was having a heart attack. An ambulance took him to the hospital, where he was told he had pericarditis. Frightening though it was, he didn’t adjust his lifestyle. A recurrence landed him in the hospital a second time, he said, when a gallon of fluid had to be drained from around his heart.
He is trying to change his habits. Sixteen days before “American Buffalo” rehearsals started, worried about being able to breathe, he quit cigarettes cold turkey. He is mostly off sugar. And he did get on that train.
On his first night in Vermont, he was in bed by 11, hours earlier than usual. The next morning, well rested, he biked to the playhouse to work on his lines before rehearsal. In the room, Mr. Rubin had the actors sit around a table reading from their scripts to get Mr. Mamet’s rhythms. But even then, just trying things out, Mr. Guirgis gave a beautifully modulated performance: tender-tough, supple, funny in surprising places.
As he starts finding his way through the play, he has been missing Hoffman, who died three years ago. “He was the guy, when I really needed help, I really needed advice, the person who I actually would value what he said — it was him,” Mr. Guirgis said.
In the back flap of the binder that holds his script, he has tucked a photo of Hoffman with his children, a reminder of the seriousness of the work and the importance of attending to the rest of life.
“I would love to call him,” Mr. Guirgis said, “and be like, ‘Bro, what do I do?’ ”