One question keeps popping up as Cicely Tyson prepares for her first stage role in more than a decade: the heroine of Emlyn Williams' classic, "The Corn Is Green." Why is she, perhaps the foremost black actress of her generation, playing an English schoolmarm in a 42-year-old drama set in Wales?

"Why not?" she snaps, her dark eyes flashing, at a break in rehearsals for the revival, which opens on Thursday in the Opera House.

At the first meeting of the cast, director Vivian Matalon preempted any discussion by announcing, "We are not doing an updated or contemporary version of this play. We are doing it in its original form. The actress in the role of Miss Moffat is Cicely Tyson, because she is a good actress. And that's all I'm going to say about it."

That's somewhat more than Tyson herself seems ready to say. She has just rebuffed the attempts of her producer, Zev Bufman, to plant a conciliatory kiss on her cheek for some undetermined slight earlier in the week. Bufman says it's only a game between them. When Tyson's miffed with him, he tries to kiss her and she pretends to shrink from his advances "for about four days"; then it's all over. Tyson turns instead to lunch, which sits inside a shopping bag from "Les Trois Petits Cochons," a fashionable charcuterie in SoHo, decides she can't "eat and talk" at the same time, relents a little and reconsiders the question people keep posing.

"I'm not being defensive," she insists. "It's just that I think I have proven myself as an actress and should be considered for roles on those grounds alone--not because I'm a black actress, which limits my ability to work."

She takes a sip of hot water, laced with honey and lemon, and relents a little more. "Some people think this is the perfect vehicle for me. I don't know why other people find it curious. Miss Moffat is a challenge and that's what gets my juices going. As performers, aren't we supposed to stretch our instruments? Look at Olivier. If a role will give you an extra inch, you should be able to do it."

Even a role that, from all indications of time, place and tradition, is that of a white woman? "I don't know if that's true," Tyson counters. "My parents are from the West Indies. Slaves were brought from Africa to the British Isles, then transferred all over the world. Who's to say that one of them couldn't have stayed on and become a teacher? I don't know that. You don't know that. Blacks have been a part of this planet, and in different parts of the planet, as long as whites have."

The stern facade is beginning to crack, allowing her majestic beauty to shine through.

Well, what about her celebrated predecessors in the role: Ethel Barrymore, Katharine Hepburn and Bette Davis?

"Oh, you just hit me in the gut. If I'd had my lunch, I'd upchuck!," she howls, doubling over in what is probably only simulated terror, but seems to indicate nonetheless a loosening up of her mood. "What an extraordinary trio. I just can't allow myself to think about it. I can only hope I'll be judged by my own performance. I guess the real question is, are audiences going to be conscious of me all during the play or will they forget about color in the first five minutes and deal with the play itself?"

Which is another way of saying that Cicely Tyson, who never has shrunk away from challenge, may be up against one of the biggest in her career.

"Without question," she says, smiling the dazzling smile of the incipient conquerer.

Tyson always has been particularly selective about the parts she will and won't play. As the title character in "The Autobiography of Miss Jane Pittman," she aged from 17 to ll0 and won an Emmy in the process. So convincing was she portraying Pittman at every step of her long life that Tyson now gives her own age, when anyone is impolitic enough to ask, as "anywhere from 17 to 110. Pick a number." Various reference books place her in her fifties, but she refuses to divulge her birth date, explaining, "I will not allow it to be the measure of whether or not I can play a role. If I get to the point I'm too old to play a role, I'll be the first to say it."

She was Coretta King in the TV documentary drama "King"; Kinta Kunte's mother in "Roots," and Chicago educator Marva Collins in yet another TV drama, "Welcome to Success: The Marva Collins Story." Although a spate of articles subsequently questioned Collins' precise success rate in educating troubled ghetto youths, Tyson refuses to join the naysayers. "When I was getting ready to do that piece, I went to Chicago and spent a good deal of time there with her. I slept in her home, ate with her, became part of the school. I would think it's extremely difficult to keep up a facade 24 hours a day. You have to break somewhere. I saw no indications of any of the things I've heard since then. I can only judge by my own experiences and they were extraordinarily positive."

"Positive" is the gauge, and her role in "The Corn Is Green" fits the pattern: a headstrong educator who has vowed to use an inheritance, left her by her uncle, to educate the sons of the Welsh miners out of the sooty poverty that seems their destiny. "Miss Moffat is a humanist first and foremost," she says, "and that doesn't necessarily have anything to do with color, race or creed."

What Tyson categorically refuses to play are hookers, deadbeats or shiftless women, although she believes she could do a smashing job at it. "When I decided I was going to be an actress, the last thing in the world I intended was to be an actress for the cause," she explains. "I expected to play any and all types of roles. But there came a time in my career when I became very conscious of the kinds of images of blacks that were being projected through the media. In the late 1960s and early '70s, that whole rash of blaxploitation films was, for the most part, unrealistic, degrading, negative. They had nothing to do with us as a people.

"Because of that, I made a conscious decision that I would not accept roles unless they projected us, particularly women, in a realistic light, dealt with us as human beings. It was a decision that cost me years of employment. I did a movie called 'The Heart is a Lonely Hunter' in 1968 , and I didn't do another one until 'Sounder' in 1972 . But I felt I had to make some very strong statements through my work."

The strong acting statements she did make resulted in her elevation to stardom, and the NAACP has bestowed upon her a total of five Image Awards as best actress. Tyson is not sure, however, that her personal crusade has wrought significant changes elsewhere. "I think I've made people more aware of what they're about," she allows. "But in terms of the industry, I don't think I've changed much. A little, maybe. That's what's so discouraging. It's so futile. Makes you want to bury your head in the sand sometimes. Show business is a business, there to make money, and apparently it is not felt that money can be made with blacks. Supposedly no one wants to see us now."

She shrugs, a gesture that would signify resignation in anyone else, but in Tyson merely seems a solidifying of her forces. "I'm not a quitter," she hastens to add. "I'm determined to survive. I figure if I've come this far . . . It's the hanging in there that counts. I'm constantly working to get work. That's what I do in my spare time. I work at getting work. That means dealing with agents and managers, looking for properties, reading books, taking an idea to somebody and asking, 'What do you think of it? Nothing? Fine!' Then you take it to somebody else. It's a 24-hour job."

Purposeful as Tyson's career is these days, it began more or less by fluke. Her parents separated when she was 9, and she was raised in Harlem by a mother who was devoutly religious and forbade her children to go to the movies, let alone the theater. "The only kind of performing I did was in and around the church," Tyson remembers. "Oddly enough, I can recall my father saying something about me going on the stage and wanting to give me dancing lessons, but my mother wouldn't hear of it. He must have sensed something that was there. I certainly wasn't aware of it. I was very, very shy. Once I was given a part in a school play--I think it was 'The Pirates of Penzance' and I was one of the maidens. But I couldn't talk. I just whispered and, of course, I lost the part. It just never occurred to me then to be an actress."

She looks grave, a state that befits her fine profile. "Who knows what our motivating force is? My mother was the one who broke from my father, and afterward he always said to her, 'If anything happens to any of my children, you'll be responsible.' So she coddled us, kept us in the nest. I was very close to my father--he always called me his stringbean--and after he left I missed him desperately. I think it was my need to please him and prove my mother wrong that may have prompted my decision to be an actress. Once I'd made up my mind, my mother just knew I was going to Hal-i-fax. As a matter of fact, she asked me to move out of the house. She said I couldn't live there and do this acting thing, whatever it was."

Tyson passed through several preparatory stages first, beginning with a stint as a secretary in a social agency. "One day, I was just overwhelmed with the mechanics of the job," she recalls. "I thought, 'God didn't put me on the face of the earth to type for the rest of my life.' There had to be something else, although I had no idea what it was. A week after that, my hairdresser called me and asked me if I'd appear in a hairstyle show she was doing. I did during my lunch hour and it was great fun."

Before long, the lunch hours were not sufficient. Tyson, with her swanlike neck and her elegant features, carved as if in rare onyx, was increasingly in demand as a model. But even that became "mechanical" after a while. Then one day, a fashion editor casually put her on to a movie role. Tyson met with the producer, got hired for "The Spectrum," and had her first taste of professional acting, before the project ran out of money.

"Once my appetite had been stimulated, it was not enough for me to scratch the surface," she says. "I knew that I had latched onto something and that it served as a sort of release for me. So I went to school and started studying. The results are as much a surprise to me as they are to my friends and family."

Happenstance was only part of it. Director Vinnette Carroll, one of Tyson's early mentors, later observed, "I was sure Cicely would make it. She wouldn't know how to be derailed." It was in fact a role in Carroll's production of "Dark of the Moon" in Harlem that led to Tyson being cast in the now legendary off-Broadway production of Jean Genet's "The Blacks."

For all the vulnerability she can project on the stage or the screen, Tyson guards her personal life with the zealousness of an attack dog. Two years ago, she married trumpeter Miles Davis and acquired several stepchildren, the exact number of which she'd just as soon not disclose. "My tendency is to protect the reality of my life. Miles is much more outgoing than I am," she says. "What I try to do, as soon as rehearsal breaks, is go to my gym or my masseuse and spend an hour leaving the work behind me. On those occasions I've gone directly home, Miles looks at me and says, 'Where's Cicely?'

"I feel very strongly that you've got to separate your private life from your professional life. Otherwise, the two merge and become fantasy. The fact that I have a private life that is real to me is what allows me to go into fantasy."

She fends off a few more questions with airy grace and infectious laughter that are no less charming for masking a resolve of iron. Then suddenly she permits herself a confession. "Miles knows that I am happiest when I am working. But this is the first time I've been engaged in a major project since we've been married. It isn't easy. He's very much into sketching now. When I come home, there is a trail of sketches strewn inward from the door, with these cartoon characters saying things like, 'Well, you've been gone all day.' 'He doesn't like that.' 'We don't like it, either.' He keeps me laughing most of the time, which is something I need desperately. I've always said if a man can make me laugh, he's got me."

On the subject of Elizabeth Taylor, who is producing "The Corn Is Green" with Bufman under the banner of the Elizabeth Group, Tyson nearly buttons her lip, however. The two of them appeared in "The Comedians" and spent nearly nine months together filming "The Bluebird" in the Soviet Union; but "there are degrees to which one can know a person." Tyson does admit that she recently saw the Taylor-Burton "Private Lives," also being produced this season by the Elizabeth Group. But asked for her reaction, she gazes distractedly at the ceiling, shifts her attention to her nails and all but whistles a diversionary tune.

"I think," she finally says, "being in a position where you're wearing two hats is extremely difficult . . . to be in."

She is saved from further comment by a trio of young black actresses in leotards, who are auditioning down the hall for "Dreamgirls" and have come to search out Tyson. Their admiration is conveyed by girlish squeals of excitement and gawky, self-conscious stances. "You're so beautiful!" one of them says. "My mother will never believe this," shrieks another.

Tyson at once becomes the role model, graceful and dignified, plying the girls with gentle questions about their ambitions, signing autographs and wishing them good luck. She knows that a decade of triumphs have made her, in the public eye, a kind of super black woman.

"Well, I would like to think I've made someone understand that just because I was born black and a woman does not make me any less a human being," she says, once her fans have left. "For the last three or four years, I've been thinking I've made my point. But maybe I haven't because otherwise when the list for leading ladies or character actresses goes out, my name would be on it. But nobody thinks of blacks in that context.

"So I guess it becomes your responsibility, once you've achieved a certain amount of recognition, to continue to pound on the door. It's a tremendous responsibility I wouldn't mind sharing, I can tell you. But it's a necessary one. Look at those girls. That's the future."

She turns her sleek head--toward the past maybe. "I can't help remembering that somebody looked at me once."