THE PART is not one most beautiful leading ladies would seek: a hunchbacked German psychiatrist. But Irene Worth is neither an ordinary nor a predictable actress, and the part of Fraulein Doktor Mathilde Von Zahnd in "The Physicists" (which opens Wednesday at the Kennedy Center) is for her a fascinating challenge.

Irene Worth is called a "classical actress," which is often a way of saying that someone is not very famous. She is, instead, well-known, with two Tony Awards, an untarnished reputation for artistic integrity and a rare, educated intelligence that informs her work.

For example, when asked what satisfaction she gets from her work, she said: "It gives me a chance to share ideas and quality in literature, the ideas of great writers." And a glance at her resume' substantiates that: She can currently be heard weekly on public radio in "The Odyssey," and she has to her credit at least nine Shakespearean roles as well as Sophocles, Ibsen, Chekhov, Shaw and modern playwrights such as Eliot, Williams, Saroyan, Beckett, Albee and Coward. She has also done an extraordinary range of things, from the most traditional to the avant-garde, from "Champagne for Delilah" to Peter Brook's avant-garde performances on a mountaintop in Iran.

"I think she's one of the three or four great actors in the English-speaking world," said Austin Pendleton, who directed her in "Misalliance" and last year in "John Gabriel Borkman." "The thing that amazes me about her to a greater degree than any other actor is that she has a powerful intellect and knows how to make that work for her emo-See WORTH, L10, Col. 1 WORTH, From L1 tionally . . . There are a lot of actors who are smart, but that often translates into an intellectual performance. She can translate it into emotion."

She lived and worked for nearly 30 years in England, with occasional forays to the United States, was a member of the Royal Shakespeare Company and was awarded a C.B.E. by Queen Elizabeth II. She sounds English, although raised in southern California, and the pronunciation of her first name changes depending which side of the Atlantic she is on. (Over there it's pronouced Iren-ee). She returned to the United States in the early 1970s to do a revival of "Sweet Bird of Youth," for which she won her second Tony.

Recently, she has been ensconced in a small apartment near the Kennedy Center, rehearsing a role in Friedrich Du rrenmatt's play that she originated in 1963. (Roger Stevens recalled that when he produced the play on Broadway in 1964, he wanted her to recreate her role. He promised her top billing. Unfortunately, his partner, Robert Whitehead, had meanwhile promised top billing to Robert Shaw. The conflict could not be resolved, and Worth bowed out. She said she doesn't remember this incident.)

She looks, this day, rather like a Georgetown matron in sweater and tweed skirt, although it seems impossible that she was born in 1916. There are books all over the apartment, which has the unmistakable charmlessness of temporary digs, and classical music playing on a radio. Irene Worth likes to talk, but not much about herself.

When she starts a new part, she said, she first reads and rereads the text numerous times. Then she does as much research as she can, where it's appropriate. But research is a somewhat misleading word for the process that she goes through, which is a process not only of synthesizing her deep involvement with music, art, literature and even architecture, but also of her observation of people in general. The raw material of actors is humanity, and like most great ones, Worth is constantly wondering at the people she sees.

This is a woman who studies women shopping in supermarkets, seeing faces "who live under the burden of just having to get through the day . . . and have to be cheerful and make the best of the situation they're in." And bag ladies. She went out with some policemen in London when she was working on a part for a television play to see how the derelicts lived in abandoned warehouses, huddled around fires and numbed by what they call "meths," a devilish low grade of alcohol. She described it as a scene out of Caravaggio. She started giving them blankets and clothes, and organizing neighbors to press for some kind of legislation, and then struggling with the question of how to help people who reject help.

"I am preoccupied with these bag ladies," she said. "I'd like to know how they got there, what final thing made them give up the activity of living. I thought about just asking them, but I don't want to impose on their privacy."

She walks the streets of New York, marveling at the architecture, and the way the sun reflecting from a particular building at a particular moment envelops the passers-by ahead of her in a radiant aura. She does readings to raise money for libraries, and spends time in them unearthing letters for her one-woman show, "Letters of Love and Affection" (which she will do here on Jan. 10).

"It's a horrible place," she said of New York. "But I like it."

"I don't ever want to lose touch with the world I live in," she said. "Or with the other arts. Diderot said he found himself rushing to the mirror to see the expression on his face the second he heard his father died. I don't think we actors need to be like that."

When she played the part of Helena in "A Midsummer Night's Dream," all the help the director, Tyrone Guthrie, could give her was to suggest she was "a little white poodle lost in the forest." Worth conceived of Helena as "a wacky dumb blond." Her Goneril was a woman motivated by love that "came out wrong," who "went mad like her father." Her "Winnie" in Beckett's "Happy Days," in which she is buried up to her waist, is considered one of the great performances.

Du renmatt described Fra ulein Doktor Mathilde Von Zahnd as "a very celebrated person, not just because the hunchbacked spinster in her eternal white coat is descended from a great and very ancient family, of which she is the last presentable member, but because she is also a philanthropist and a psychiatrist of enormous repute . . ." The first time she played her, Worth was at a loss until she recalled the image of the pianist Clara Haskell. "She was one of the great woman pianists. When she was young she was frightfully crippled by tuberculosis of the spine. She also had to decide whether to be a concert pianist or a mathematics genius . . . She had a magnificent head of white hair, and when she played she was so pure, so concentrated."

The man who made her wig for that production is making her wig for this one, a symmetry that pleases her.

Pendleton said she was a "relentless perfectionist" who was rarely satisfied with her performances, which he described as "the best acting that could possibly be done." Her agent, Milton Goldman, says she has worked "constantly" but has chosen her projects so that "she only works with the best." She has recently done two movies, last year's "Eyewitness," and the soon-to-be-released "Deathtrap," in which plays the psychic Helga Ten Dorp.

"My dream in acting is to find a way that is so simple and so natural that the adults in the audience have the excitement we have as children," she said. "Imagine an audience seeing 'Hamlet' and not knowing what is going to happen next. That has to do with immediacy, with urgency. That is what every play should have."