ONE OF the theater's most enduringly obnoxious traditions is the practice of applauding a star's entrance. A play should begin with a clean slate. Applause, if it is to mean anything at all (and it has been meaning less and less lately), should be reserved for the here and now. Our mere attendance ought to be tribute enough to a participant's past triumphs.

This is an easy rule to frame, but hard to follow when the star is Mary Tyler Moore. It has been three years since 30 million regular viewers said a sorrowful goodbye to Moore and the WJM-TV newsroom in Minneapolis. No frilendlier room, no more colorful crew of inhabitants and no more widely admired leading lady have come on the air since.

Just the same, Moore may be a bit embarrassed by the convulsive clapping that greets the first sight of her in "Whose Life Is It, Anyway?" at New York's Royal Theatre. This is also, after all, the first sight of her on a Broadway stage. (A musical version of "Breakfast at Tiffany's," in which she co-starred with Richard Chamberlain, colosed on the road.)

In "Whose Life?" she is confined to a hospital bed with a ruptured spilne, unable to mobilize any muscle below her neck, and what few powers she can muster she uses to beg her keepers to let her die. The folks out front, meanwhile, are remembering Mary Richards, Rhoda, Phyllis, Mr. Grant, Ted Baxter and the rest of the MTM Productions gang. There even comes a moment in the play when Moore, quite by coincidence, reads a line with the same gesture of resignation that was her TV trademark. This, naturally, earns her another thoroughly irrelevant wave of applause.

How does an actress break through such a wall of familiarity?

Through effort and talent, and never completely. Moore wastes no time knocking the stuffing out of the theory that she can't cut it as a stage actress, outside the carefully packaged world of TV sitcoms.She has a strong voice and a strong stage presence -- despite the temporary loss of her limbs. But clearly when an actress becomes so familiar in one role and one context, she creates vibrations that will forever interfere with her attempts to do anythine else. By final curtain time, nevertheless, she makes her character's unwavering determination to die affecting whether you side with her or not. The audience applauds again -- and then the timing is right.

In Brain Clark's original version of the play. Moore's part was played by the English actor Tom Conti. This transsexual substitution -- accompanied by a shift of setting from England to America -- is a heartening display of good old-fashioned showmanship on the part of producer Emanuel Azenberg. Sex is immaterial, say Azenberg and Clark: The play is about the right to die. And it is hard to argue with them. "Whose Life?" is not only issue-oriented but issue-fixated. aIt sticks to its subject with the commitment of a cat that has cornered a mouse. Sex is a factor only in a few scenes when the hero flirts with the hospital staff. And it was easy to change the principal attending doctor from a woman to a man (although a team of male nurses might have seemed unrealistic, so they go on being female, at some expense to the banter between them and their patient).

Conti had one quallity in the part that Moore lacks -- a mad, manic delilvery that made his mouth seem to fly like one of those little metal balls in electric typewriters. Moore's performance is more easygoing, less acidic. But the differences are less pronounced than the similarities.

"Whose Life?" remains a theatrical rarity -- a play of ideas. And in between its blasts of wit, it remains, under Michael Lindsay-Hoff's ponderous direction, as stuffy as a semiprivate hospiltal room. With Conti in the lead, "Whose Life?" was a medium-sized success that opened and closed last year. It is six of one and half a dozed of the other whether to be greatful for the pleasure of seeing Mary Tyler Moore on a stage, or to point out that the play, like its hero/heroine, has been kept alive artificially and had a right to die.

Judd Hirsch, the star of TV's "Taxi," is a stage actor of long standing, and no one could be righter for the part of Matt Friiedman, a homely Jewish accountant from St. Louis, circa 1944, and the hero of Lanford Wilson's "Talley's Folly," widely regarded as the most serious worthwhile play or the most worthwhile serious play of the Broadway season.

Plays hardly ever send us a clear signal anymore that they are ready to begin, and "Talley's Folly" is no exception. As we enter the Brooks Atkinson Theater, the curtain is already up and the stage lights already aimed brilliantly at a striking set -- a sprawling, gazebo-like boathouse shrouded in trees and vines, enfolding what looks to be a genuilne lilycovered inlet where a rowboat seems to rock with the tide. Then Hirsch comes bounding down the aisle and starts talking to the audience, with the house lights still lit. We are here, he tells us, to watch him woo and win a young lady. Barring the unforeseen, it should be a "piece of cake," he says, and the play will last a mere 97 minutes, without intermission.

In the midst of this peculiar preface, he interrupt himself. "I know what you're thinking," he says. "You're thinking if I'd've known it was going to be like this, I wouldn't have come."

I was thinking something like that, and still am. I'm thinking that this is one very dumb way to start a play. Hirsch is speaking more for the playwright than for the character here, and what the playwright is saying is the theatrical equalivant of "Look ma, no hands!"

As soon as Hirsch turns his attention from the audience to co-star Trish Hawkins, "Talley's Folly" gets its act in gear, and the transition is considerably eased by some stunning sound effects -- frogs croaking, dogs barking, crickets singing -- that take full advantage of the last decade's advances in audio technology. The play itself uses Matt's courtship of farrmer's daughter Sally Talley to portray the gulfs that divide Americans different cultures, and the bridges that can span those gulfs. It is a funny and a lyrical play, and Hirsch gets maximum mileage from it, but the final minutes of "Talley's Folly's" promised 97 are extremely monotonous. Instead of developing his characters and their purposes, Wilson builds his play toward another of those ttiresom revelations of a potent secret in a character's past that playwrights love so. As often happens, the disclosure connot match the expectation, and the mounting obsession with the past makes the present tedious and the future a matter of complete indifference.

Hirsch's performance lingers in the memory, but so does a question: As Matt Friedman, he speaks with a strong Eastern European accent and does a series of letter-perfect imitations of various downhome midwestern types. His skills as a mimic are impressive, but would a man with such an accent have such a gift? Would a man with such a gift have such an accent? Or is the actor, like the playwright, doing a little boasting at the expense of the play?

Yet another TV star, Louise Lasser of "Mary Hartman, Mary Hartman," has top billing in "Marie and Bruce" at the Public Theater. She also has a wonderfully zapped-out quality that makes her an ideal choice for this role, or for any prototypical victim of the mid-20th century's cruel forces. Her low-keyed, bliitzed-out delivery is delightful even as it threatens to put us to sleep -- and the same can be said of Wallace Shawn's highly original but anarchiically repetitive play.

"Marie and Bruce" is a guided tour of a day in the life of a Manhattan marriage, with Lasser -- a much larger Lesser than in her "Mary Hartman" days -- as the guide.

Her husband is the hopelessly slight, outmuscled figure of Bob Balaban, who is the constant object of her expletives, ranging up from "pathetic pig" to a series of fouler nouns and modifiers aimed at his body, his clothes and his friends.

"I'm only a person," says Balaban with a meekness so mild that he ought to inherit the universe, not just the earth.

The action is played out on a turntable that revolves us through an entire day's worth of this abrasive nonsense. This is a marriage with all the normal rhythms and without even the most perfunctory intrusion of a serious isssue. We accompany Marie and Bruce to an artsy New York cocktail party peopled by a wonderfully vacuous crew of guests, and we wind up at a Chinese restaurant where she tells him: "You're one of the least interesting people I've ever met in my whole life."

"I know you're unhappy," Bruce replies callmly, "but we can discuss your unhappiness and still have a very nice evening."

"Bruce!" she screams back at him. "Don't you know that you're not a living person?"

"I thought I was one," he says.