Making this production even more poignant, Cooley has been losing his hearing since he was 14.
Cooley says the play is two plays in one: " Tribes speaks about all families. Deafness is only a small part of the drama. It's about families, how they work or don't work, and what brings them together."
During rehearsals, Cooley-as-Billy picks up on almost all of what's happening around him and responds in normal speech. But when the crosstalk gets to be too much, his eyes go to Don Rubel, a professional signer. They exchange rapid-fire signs to clarify.
"I've interpreted about 200 plays and musicals," Rubel says during a break, "but this is different. To hear all the actors' discussion and [director Stuart Carden's] feedback - it's amazing. It helps my skills."
This rehearsal is a volatile family's rough-and-tumble, an intellectual You Can't Take It With You played out at a long dinner table strewn with meals and snacks. And director Carden, who is associate artistic director of Writers Theater of Glencoe, Ill., has a dual task: To direct the ensemble, and to take care of Cooley.
Many of his directions - "We all need to take a little bit more care of giving face time, of the face time Sylvia gives to Billy, and of the face time we're all giving to Sylvia" - do both at once. He repositions boyfriend and girlfriend so that Billy - and also Cooley, the man playing him - gets the best benefit of the interplay. Sylvia is "very much between two worlds" and "very much aware that Billy needs to see her lips." (He is an accomplished lip-reader who has never learned to sign.)
Carden and the rest of the ensemble check repeatedly to make sure Cooley is with them. (You might say they make sure to include him, something Billy's family is so bad at doing.)
"It's a very delicate process," Carden says. "Tad has real-world experience of going deaf, and I'm a hearing person who could never comprehend fully what that means." He says he has asked Cooley about his experience, often in private, to help him manifest the character of Billy.
Cooley is more than equal to it all: "I love it when Stuart grabs stuff out of my life and makes it more personal. It makes it easier for me to be Billy."
It's been quite a life so far. Cooley, 20, is from Vanderbilt, Texas (population 395 as of the 2010 U.S. Census). At 13, "I was asked to be in this silly talent show - and I loved it. Up to that point, nothing had really made me happy."
A year later, he started noticing a decline in his hearing, a decline that has continued: "Last January, I started learning sign language, because I need it now. My speaking voice hasn't been affected yet, and whether I'll go totally deaf, I don't know. But I'm in a transition a lot like Sylvia's."
He also moved to New York City to study at the New York Conservatory for Dramatic Arts. "It was a big leap some of my family didn't agree with," Cooley says. "But I am loving it."
Raine has written that Tribes arose out of a realization that all families are tribal. As Carden puts it, "What happens when you discover the tribe you're born into isn't the one you really belong to?"
The section they're rehearsing this day is both funny and painful. Sylvia is self-conscious, Billy caught between, and when Christopher, the clueless, pontificating paterfamilias, describes sign language as "basically, broken English," Sylvia comes awkwardly to its defense.
Cooley marvels at how well Raine has grasped his experience: "The way Sylvia describes going deaf at the end of the play, I totally and completely felt she was speaking from my heart. Any deaf person can relate to it."
Then, after a beat: "Anybody who's ever been in a family."
Through Feb. 23. Philadelphia Theatre Company at the Suzanne Roberts Theatre, 480 S. Broad St. Tickets: $46-$59. Information: philadelphiatheatrecompany.org, 215-985-0420