NEW YORK — Gabriel Byrne spends much of his new drama planted in a chair, engaged in dialogue with whomever occupies the sofa facing him. He has plenty to say, but what distinguishes his performance is something else he must do: express the act of listening.

"I found it very challenging," says Byrne, "to play a character who has to listen."

Byrne plays Paul Weston, a busy psychotherapist. The series, HBO's "In Treatment," is built around sessions with a round of clients who challenge Weston and, maybe, drive him a little crazy.

"In Treatment" is an extraordinary drama, and not just thanks to Byrne. With an ambitious structure likely unprecedented on American TV, it unwinds in half-hour episodes, Mondays through Fridays, for nine weeks — with each weekday's installment featuring the client or clients who routinely see Weston that day.

Premiering today, the series finds him meeting with Laura (Melissa George), a beautiful young anesthesiologist who wants more from Weston than therapy. Tuesday, he's in session with Alex (Blair Underwood), a brash Navy pilot who's recently been grounded. Wednesday's patient is 16-year-old Sophie (Mia Wasikowska), an aspiring Olympic gymnast who may or may not be suicidal. Thursday, he has couples counseling with Jake (Josh Charles) and Amy (Embeth Davidtz), who are bitterly feuding over whether she should terminate her pregnancy.

There's more: Paul's home life and his marriage to Kate (Michelle Forbes) are in crisis. No wonder he is driven to seek counsel from a former colleague, Dr. Gina Toll (Dianne Wiest), where he can be found on Friday's episode.

It's an ingenious format, which, theoretically, allows the viewer to focus on a patient of particular interest and follow that story line weekly, spared from overcommitting to a five-nights-per-week regimen.

But the truth is, the series may prove irresistible to a viewer in its fullness. "In Treatment" has the allure of smart drama spiked with juicy eavesdropping. And as further reward for the devoted viewer, the seemingly distinct daily narratives begin to seep from one to another — collecting in Weston's agitated psyche, where the real story resides.

From his leather chair, Weston offers each client a reassuring calm (or is it world-weariness?). In his soft Irish brogue he speaks with great deliberation (or is he distracted by issues of his own?).

No less demonstrably, he listens.

"There are very few roles that require such an amount of reaction," says Byrne, the picture of wry soulfulness in a dark suit and pink shirt, during a recent interview. "Every movement I made had to be for a specific reason. It was interesting to be so pared down to so few dramatic choices. I decided not to wear glasses, to not rely on that prop. No mustache, no pretend accent. All of that gives you something to hide behind. I decided I'm just gonna be naked here, to see what would happen if there was nothing between me and the 'patient.' "

Byrne proves equal to the task. Born in Dublin 57 years ago, he is an accomplished character actor with leading-man magnetism whose many films include "Miller's Crossing," "The Man in the Iron Mask," "Stigmata" and "The Usual Suspects."

He added to his impressive theater credits two seasons ago for a Broadway revival of O'Neill's "A Touch of the Poet," his enormous role serving as a warm-up, he says, to the 1,000 pages of dialogue he was obliged to learn during the five months "In Treatment" was shooting last year.