The closer you approach the mystery of your art, the more you become part of the mystery.

"I just chip away the stone I don't want," said Michelangelo.

"It's the writing," says Jessica Tandy. "It's all on the page."

The actress has been on a tear the last two years, making one movie after another when not appearing from Minneapolis to Broadway with her husband, Hume Cronyn, in the modestly successful play "Foxfire."

Sometimes the movies are bad, like "Stab" and "Honky Tonk Freeway," to which she added a bit of class with her sketch of a lady who orders five martinis at a crack.

Sometimes she is a wife, as in "Garp" and "Best Friends" (though for the latter they gave her a different husband), sometimes a mother, as in "Still of the Night," where she plays the psychiatrist mother of psychiatrist Roy Scheider, a deliciously understated cameo with exactly the right, slight tension.

"You just have to trust the script," she insists during an interview--solo, not the conventional husband-wife number--in her New York hotel apartment. "People aren't aware that you cannot give a good performance if your material is bad. It has to be there."

Well all right, what about "The Gin Game," which many critics berated as mediocre, though it won the Pulitzer Prize?

"It was there! Oh boy, it was there!"

That was one of those two-person plays, just Tandy and Cronyn playing gin and making remarks. Maybe it was all there, in the lines, but not everyone would find it.

When you point this out, she starts talking about directors.

" 'Honky Tonk' didn't do terribly well, but it meant working with John Schlesinger, and that part of it was fun. Then it was Robert Benton with 'Still of the Night,' and Meryl Streep and all, and you don't say, 'No thenk-yew.' I don't know much about film, and I really like to be told a lot, I'm really very much at sea."

Yes, of course. Very much at sea with 52 years of professional acting experience and a film career that goes back to 1944.

You don't talk to Jessica Tandy about acting. You watch her act.

The lovely, leafy scrim pulls up, revealing the superb Appalachian farm set of "Foxfire." She's on the porch, talking desultorily with her husband, Cronyn. They are mountain people, the accents rural. Ordinary talk. Silences. She uses silences the way Henry Moore uses holes.

Gradually you realize that the husband is long dead, seen only by her. Somehow this conceit manages not to seem ghoulish or pathetic or funny, but just a small eccentricity in a perfectly sane old woman. Is it something she does that brings it off?

People come by: a land developer, a young schoolteacher and her son, a country singer. With each one she seems subtly different. She goes to hear a concert by the son, who happens to be Keith Carradine, and we get an impression of his style, typical amplified show-biz country-western. The play seems to be rambling.

But as Act I ends, the son is back at the farm that night, and almost absent-mindedly he begins singing another way, the way he used to sing as a kid. It's a folk song, modest and casual, gentle. And Tandy leans forward a little in her rocking chair on the porch, listening. Just that little motion.

And suddenly the whole thing comes together, magically, with a power you didn't dream was there.

We are talking about "Anastasia," a rather schlocky play distinguished by one shattering scene between the young woman who really is the murdered czar's daughter and her grandmother, who believes her to be an impostor. This time Tandy says it's not the actors (or writers or directors) who make the scene work . . . it's the audience.

"The author had built the situation so that when you got to the big scene, the audience was creating an awful lot of it. There was very little that had to be done . . ."

She does add that this is getting harder and harder to do, because today's television-trained audiences seem unable to concentrate. "They talk to each other and rattle their candies. They're just not willing to give you their undivided attention because they're not used to it when they watch the box at home."

Still, even a distracted audience comes expecting to be told a story, and that is something to build on. There is no such thing, she says, as sheer acting, all by itself.

"Don't let anyone ever tell you that you can make a big moment by reading the telephone book."

She tells about the time someone asked Ethel Merman, standing in the wings just before going on in a new musical, if she wasn't terribly nervous. "Heck no," she boomed. "I know what I'm gonna do. It's the audience that should be nervous. They don't know what to do."

People watching Jessica Tandy never have trouble concentrating. The fact is they are mesmerized by the glances, the pauses, the little messages passing between her and Hume Cronyn in this quiet play.

The old woman is leaving the farm. We know that she hates goodbyes. She keeps busy collecting her things, refusing to see her husband standing there watching her. The son comes up the path for her. She is not looking back. There is a pause. No words. There is not a sound in the house, not a cough, not a breath. Then she starts on down the hill.

"A lot of people come to see us work together: We're an old married couple and I think it intrigues them. They invest us with something, look at us as some kind of dinosaurs who made marriage work. They keep asking, 'How do you manage?' Not only the older ones, even the kids ask. What can you say?"

She smiles and lifts her open hands and lets the 40 married years trickle through like gold dust.

"I tell them, 'One day at a time.' You expect to have the ups and downs, to give way in certain situations, to roll with the punches. I think it's tough, and you have to go in knowing it's tough. But there are enormous rewards. As someone said, it's not the times of ecstasy that hold you together, it's the really tough times you've survived."

She was born 73 years ago in London, played in family theatricals with her brothers "and never outgrew it," did everything from Shakespeare to "Alice Sit-by-the-Fire" in London and, after her 1930 Broadway debut, acted increasingly in this country. She has a daughter by her eight-year marriage to actor Jack Hawkins, has a son and daughter with Cronyn.

The plays and movies and TV appearances cover the theatrical landscape from Molie re through Chekhov, Shaw and Noel Coward to Edward Albee, the roles ranging from Cordelia in Gielgud's "Lear" to a pair of lips in Beckett's "Not I" (she was encased in a box that showed only her painted, spotlighted lips for this monologue which incidentally won her an Obie).

Eleven times, she and Cronyn have played together. They were both nominated for the Tony for "The Gin Game," and she won it. Her first Tony was for creating Blanche du Bois in "A Streetcar Named Desire," surely one of the greatest American theater characterizations, before which Vivien Leigh's more famous movie version pales.

" 'The Fourposter' was the first time we acted together. It was nerve-wracking. Also, the script needed a lot of work, so we had to keep changing it. Scenes rehearsed and thrown out. Costume changes from the skin out, even hair changes. We knew we had something, but there was a lot of trial and error."

They don't improvise, don't fiddle with the script (though Cronyn co-wrote "Foxfire"), don't insist on going their own way.

"We don't talk about it much outside of rehearsal. I hope directors will always direct us. What we think onstage isn't at all what's being seen out there, and we need that monitor. Once in a while, if a play's been going a few weeks, I might tell Hume, 'I think that, uh, slipped a bit there,' and he may agree or not. But rarely.

"The reason we can live and work together is that in no way do we threaten each other. We're safe: I can't play him, and he can't play me. That's basic. I think it would be terribly hard if Hume were doing marvelously and I was just dragging along behind. Because I'm ambitious in my own right. We've both always done separate things on the stage and in the movies."

They had been married nine years before they tried acting together.

"I don't always want to do two-character plays. They're an enormous responsibility. A long evening. And when you have a long run, you have to psych yourself up into thinking you've never heard what that person said to you before, and you've never said this before. If you let your mind wander, it doesn't work. It'll sound stale, and the audience can tell."

The recent rush of movies was essentially a way of keeping busy between versions of the new play, as it progressed from Stratford, Ont., to Minneapolis and on up the line to Broadway. She doesn't expect it to happen again for a while, at least not until "Foxfire" quits.

"This is all I can manage at the moment," she says. Meanwhile, Cronyn is co-writing a TV movie for Jane Fonda based on "The Dollmaker."

It is a compact, comfortable life, hardly a dozen blocks from the theater. They moved to the city after a recent bizarre incident when burglars invaded their exurban farmhouse, tied up Cronyn and his secretary and left them to escape as best they could.

Sitting in the light-filled, yellow-walled living room, composed and slightly on-guard, she fields the questions neatly and quickly. What is, then, the essence of good acting?

"If you find out, let me know," she laughs. She is right. Nevertheless, she tries. Something about spareness, she says. Less is more. She is talking about those silences you get now and then in a play. "The most wonderful flattery," she admits.

But she can't talk about it, really, that moment in the theater when time suddenly is suspended, and no one moves, and you sit there forgetting to breathe, and you seem to be right up on the stage with her, thinking her thoughts, feeling what it is to be inside that person, the person she has just created for you. Feeling changed.

You couldn't talk about something like that. You had to be there.