The cells were just some masking tape on rough office carpet, but they were cramped and claustrophobic all the same. Squares, about 5 feet by 5 feet, that the four actors were not supposed to leave.

Sarah Shourd was sitting on the floor before them, in a practice space in South San Francisco. She was dressed in all black, folded in on herself, tangled and tight. Her speech was dulcet and direct.

“So this is going to be a period where no one is communicating with each other,” she said. “I’ll let you know when I want it to stop.”

They began.

They read and they wrote.

They prayed on their knees.

They cried and they counted and they paced and they were still.

And they did it all over and over again, until Shourd called time.

“So that was 15 minutes,” she said. “And before we dive into the nuances of what we learned and what you discovered, just think about how long your character had been in that cell.”

Jake for seven years; Victor for three; Ray for 19; and Rocky had just got there. Every hour — save one — for every day for every year.

The actors were rehearsing for “The Box,” a play about solitary confinement, a subject Shourd knows well. She was one of three hikers who were arrested by Iranian authorities in 2009 while hiking in Iraqi Kurdistan and unknowingly crossing into Iran. Shourd was held in solitary for 410 days.

After her release, Shourd, a journalist and artist, spent three years researching solitary confinement, traveling the United States to talk to prisoners, trading letters back and forth. In 2016, she presented the first iteration of “The Box.” Now she is revisiting the project and staging it on Alcatraz Island for two sold-out performances on Friday, June 14, and Saturday, June 15. The piece is part of “Future IDs,” an ongoing art exhibition that seeks to use one of the largest symbols of incarceration as the setting for projects seeking criminal justice reform.

This time around, the play, has been shortened, and the set will be considerably sparer than the first iteration at San Francisco’s Z Space. But Alcatraz opens the play up, and the play changes the island. “It’s tempting to come to a prison that’s a historic monument and think of incarceration as something that’s from the past.”

“The Box,” produced by the Pulitzer Center, follows four inmates on one block as they cope with living in a world that is only as big as an elevator. Shourd goes to lengths to show the ways in which people cope and break under these circumstances, the ways a system like this strips everybody of their humanity — the prisoners and the guards.

The United Nations recommends no more than 14 days of solitary confinement. Anything more they describe as “torture or other cruel, inhuman or degrading treatment or punishment.” California recently passed reforms to limit use of the method. Now the state’s cap is two years.

That’s just California. It’s nearly impossible to get a figure on the number of people held in solitary confinement on any given day in the United States. One 2018 study suggested the number might be 61,000. That estimate was conservative; others have put the number closer to 80,000.

“So, Jake, what did you discover?” Shourd said. She called the actors by their characters’ names.

“My hands look like my mother’s,” said Jake, who is played by Lawrence Redecker.

“OK. You never noticed that your hands looked like your mother’s?”

“No.”

“I was like, ‘Oh, he discovered his hands. That was quick,’ ” Shourd said to Redecker. “Hands were big for me. Hands were huge for me. Hands are friends.”

Victor, played by Carlo Aguirre, found himself re-creating memories. He held a blanket as he once held his newborn baby girl. Ray, played by Dameion Brown, kept himself busy cleaning his cell and stilling his mind.

“Thank you guys for trusting me, by the way,” Shourd said. “You had no idea how long that was going to last.”

Brown’s cell, his real one, not the one made of tape, was 5 feet by 7. He lived there for a year, in solitary, from about 2002 to 2003. (Exact timing is hard; the whole thing “was kind of blurry.”) In his cell was a metal rack — “a bunk” — with a thin, plastic-covered mattress that “really provided no cushion or comfort at all.” The toilet was attached to the sink. Sometimes there were bars; sometimes you just got a closed door with a feeding portal.

As of August, Brown will be four years on the outside. “It took some time for me to release that yoke,” he said. “I was able to rebuild my mind from the weight of that. … It did take time, it did take deliberate effort, and there are things you’ll never forget.”

A couple years back, Brown saw the original production at Z Space. “I appreciate how close to truth it was … the boldness with which she approached it.” So, he auditioned for Alcatraz. “We had this belief, if people on the outside knew what they were doing, they wouldn’t be allowed to do it. You don’t completely believe that. But it was something that you hold to, hold on to so that you can believe in humanity.”

Performing in “The Box,” he said, is a way to make good on that hope, to see if it was worth anything — “an opportunity to present it properly to the outside.”

“The Box”: Drama. Friday, June 14, and Saturday, June 15. Both performances sold out. Alcatraz Island, S.F. www.sarahshourd.com