New York ? When Fiona Shaw talks about acting, it pays to listen.

“I am a great believer in language and the rhythm of language,” says the Irish-born actress, now giving the best performance on a Broadway stage this season, “and in the rhythm of the play.

“I think Americans often talk about acting in terms of character, but we don’t,” explains the woman who is inhabiting a modern-dress “Medea,” unnervingly portraying the vengeful fury at the center of Euripides’ bloody Greek tragedy. “We talk in terms of scenes.”

On stage at the Brooks Atkinson Theatre, those scenes are startling, even terrifying. At one recent performance, an audience member reportedly needed medical assistance when things got a bit intense.

But then Medea is the kind of woman who plots the death not only of her husband’s new paramour, but her own children as well. These murders are graphically flung in the face of the audience after a savage showdown between Medea and Jason, her unfaithful husband, played by Jonathan Cake.

“It’s physically terribly tiring — everything aches. It’s quite like ballet or opera or a bit of both,” the actress says of her nightly battles.

“The bigger the challenge of a play, the bigger the leap into the imagination. I’m shattered now. I feel like I have done 10 rounds. And that takes until about 3 or 4 o’clock in the afternoon to recover from. And then it’s time to start all over again.”

Taking chances

Shaw finds the language of “Medea” full of vacillation — “I must do it; I can’t do it,” the actress murmurs, slipping eerily and just for a moment into character. “That tells you a lot how to play the role.”

Shaw is the kind of actress who takes chances. She leaps into the darkness and, more often than not, soars. Her resume for the last two decades is stuffed with choice roles, in plays ranging from “The Rivals” to “Machinal” to “The Merchant of Venice” to “Electra” to “Hedda Gabler” to “As You Like It” and more for such distinguished troupes as the National Theatre, the Royal Shakespeare Company and Dublin’s Abbey Theatre.

In 1996, she appeared on the cold, cavernous stage of New York’s Liberty Theatre, one of the crumbling playhouses a block of West 42nd Street before it was transformed into a glittery, neon thoroughfare. Her 37-minute vehicle? Reciting “The Waste Land,” poet T.S. Eliot’s monumental meditation on death and resurrection.

Actress Fiona Shaw poses in her apartment in New York. Shaw plays the title role in the Greek tragedy Medea, which runs through Feb. 22 at Broadway's Brooks Atkinson Theatre.

Yet Shaw is more likely to be known here for her film roles in such diverse movies as “My Left Foot,” “Three Men and a Little Lady” and the first two Harry Potter extravaganzas in which she portrays Harry’s grudging Aunt Petunia.

Tragedy and humor

“Medea” grew out of her close collaboration with director Deborah Warner, who has directed Shaw in many of her major projects, including the title role in Shakespeare’s “Richard II.”

Warner was sitting in an airport lounge reading a tough, modern translation of “Medea” by Kenneth McLeish and Frederic Raphael. “The central spine of the Jason and Medea scenes just leaped out at me because I think I was in the mind of wanting to do a much more contemporary play,” she says.

“The piece I longed to be doing at that moment was ‘Who’s Afraid of Virginia Woolf?’ — which I will do one day with Fiona but which she is far too young for now. I want something with that amount of wit and that amount of power.”

Shaw concurs.

“We were very keen not to do a Greek play,” she says after having already done a highly praised “Electra.” “And then this translation was the nearest thing to an American play. The scenes between Jason and Medea are terribly like Edward Albee or Tennessee Williams.”

Both director and star were struck by biting, often bitterly funny remarks Medea hurls at her duplicitous spouse.

“I think tragedy should be full of humor,” Shaw says. “And, of course, comedy should be full of tragedy. I know that in one’s own life, moments of greatest gravity are also potentially the funniest. There’s nothing politically correct about life.”

“Medea” began three years ago in Dublin for a six-week run at the Abbey Theatre. Then Shaw and Warner put it to bed for 18 months before recasting it for a three-month London engagement.

After a year’s hiatus, they returned to the play in 2002, bringing it first to the Brooklyn Academy of Music, then other American venues before coming to Broadway for a run through Feb. 22. An engagement in Paris in March follows before Shaw turns her attention to a third Harry Potter movie.

“All plays only have a certain life. I would think that this is the end of it,” she says. Quite a run for a production Shaw and Warner originally didn’t want to do at all.