NEW YORK — Gabriel Byrne looked exhausted. Three days earlier, he had wrapped production on the second season of "In Treatment," the HBO drama in which he plays therapist Paul Weston, and as he sank into a sofa at the network's midtown headquarters, the Irish-born actor still had heavy bags etched under his eyes.

"It's a doggedly difficult role to play," said Byrne, who is in every scene of all 35 episodes, which begin airing tonight. Each was filmed on a Queens soundstage in about two days, a punishing pace that forced the actor to master his lines breathtakingly fast: by the end, he was up to 12 pages in 30 minutes.

"I didn't experience the winter in New York at all," he recalled. "I arrived in the dark and I left in the dark, and all day I was in a room in a chair. It taught me an awful lot about perseverance and stamina and focus and concentration, and not staggering under the weight of it."

It was a grueling experience to undergo for a quiet drama that attracted a small audience in its first season, despite its many critical plaudits, which included a Golden Globe win and Emmy nomination for Byrne.

But the actor speaks about the series in ambitious terms not usually applied to a television show. "The themes that are examined in this season are reflective of our culture, our society, in a larger context," he said. "In my opinion, it deals with the loneliness of the kind of communities that we live in. Everything that we took for granted, the stable pillars of society — that's no longer there. So uncertainty produces fear and anxiety."

This season finds Paul coping with the aftershocks of his divorce and an ill-fated relationship with one of his patients. Having relocated from Maryland to Brooklyn, he is rebuilding his practice with a new batch of patients: Mia (Hope Davis), a successful lawyer who longs to have a baby; April (Alison Pill), a young architecture student diagnosed with cancer; Oliver (Aaron Shaw), an 11-year-old caught between his divorcing parents; and Walter (John Mahoney), a powerful executive dealing with a professional scandal. As he tries to navigate their issues, Paul also wrestles with the fallout from the death of Alex, a Navy pilot he was treating last year. For help, he turns again to Dr. Gina Toll (Dianne Wiest), his one-time mentor with whom he has a contentious history.

Each episode plays out like a half-hour, one-act play featuring Byrne and another actor, just sitting in a room, talking. The drama builds through the slow reveal of the real trauma behind the patient's turmoil.

"There's something about seeing people bare their souls and seeing that this person you're baring your soul to has just as many warts as you do — it's fascinating to watch," said Mahoney, whose last television role was playing Martin Crane on "Frasier." "I had no intention of doing another series, but you get offered a part like that, and you just don't turn it down. It was exhausting and exhilarating at the same time."

The unusual series, based on a popular Israeli show, captured a small but devoted following in its first season. On average, a little fewer than 2 million people watched each episode during its multiple plays on HBO channels and on-demand, according to the network.

The way "In Treatment" was scheduled, with a new episode debuting each night of the week, may have been daunting to viewers, said Sue Naegle, HBO's president of entertainment.

In an effort to attract a larger audience, this season's premiere episodes will air in two blocks on Sunday and Monday nights.

"I hope people watch it, because I think it's extraordinary," Naegle said. "But even if the numbers don't improve greatly, it's still a success for us. When people like this show, they're obsessed with it. Because of Gabriel's performance, they feel a real intimacy with him. It's voyeuristic, and you feel you're part of the experience."

Production this season was moved to New York from Los Angeles, in part at the insistence of Byrne, who wanted to be closer to his children. And Warren Leight, a playwright and TV writer who most recently helmed "Law & Order: Criminal Intent," took over as executive producer from Rodrigo Garcia.

Leight said he decided to make this season more of an adaptation of the Israeli series rather than a strict translation: "Maybe because last year they were so faithful to the Israeli version, they lost the opportunity to find characters that are as relatable to an American audience."

He tweaked April's back story, giving her an autistic brother instead of a manic-depressive one, and excised the societal pressure to have a child felt by the Israeli version of Mia, a Sephardic Jew in the original iteration.

Although each patient is wrestling with different issues, a fraught relationship between fathers and children links the various stories together. "What amazed me is watching the crew," Leight said. "When we got to episodes where fathers were talking about losing their sons to divorce or losing their own dads, you'd see these 55-year-old grips weeping at the monitors. I thought, 'OK, good.' "

For Byrne, the role gave him the chance to explore masculinity in a way other than that usually portrayed in contemporary culture.

"There's no posturing with him," the actor said. "He's intensely vulnerable, and he is a very committed professional, but he's a damaged human being, and he struggles with life, and he's engaged with his own weaknesses. I like to examine that, as opposed to the stereotypical notion of the strong man who knows the answer to everything and knows how to act in every situation."

But such an intense part took its toll. "Listening on-screen is tremendously difficult and really de-energizing," Byrne said. "And listening is what this role is really all about: showing you're listening, and yet not over-showing that you're listening. Because the big trick of it is to try to let the audience see what you're thinking, and not let the patient see what you're thinking."