Grinning in glee as he heads across the restaurant toward his table, the debonair Michael Tilson Thomas announces his apology for Mozart:

"I just discovered that Mozart made a mistake!"

Of course, it wasn't a very big mistake, but, once seated, Tilson Thomas eagerly leans past the goblets, score in hand, to point out the offending passage in the 34th Symphony. He is conducting it here four times with the National Symphony, the final concert tonight.

"There it is! A Mannheim figuration, but here in the recapitulation, in the coda, in fact," asserts the delighted conductor, "he runs into a conflict." Tilson Thomas sings the passage -- rather well, in fact, and well within earshot of neighboring diners. He seems to be conducting himself with his free hand.

"You see, there is this very basic harmony change on the half bar. Now he wants to continue that here. But he's also got this rhythm going, and he wants to keep it. But that's a complete clash, because this theme is based on the idea that the harmony change happened on the fourth beat, so what you've got here is the tonic and the dominant simultaneously."

How in the world could Mozart have done it? After all, Mozart's music was the only thing about his life that was not utterly disorderly.

"Well," says the conductor in a forgiving tone, "I think that what it is, is that the guy wrote this stuff so fast. It's beyond comprehension."

Ever since that day in October of 1969 when at age 24 he leaped from obscurity to take over the Boston Symphony mid-concert from the ailing William Steinberg, and ended up directing much of the subsequent season, Michael Tilson Thomas has been nothing if not lively. His agility in delivering a luncheon exegesis of Mozart while awaiting arrival of his Campari and soda is comparable to the stylishness of his body English as he whips 100 or so players into a cohesive unit -- something he has done brilliantly in his recent concerts with the National Symphony.

Since his famous debut there has been little debate about Tilson Thomas' technical or intellectual command -- the question has been the course of the career rather than its quality. The breadth of his interests is remarkable -- his repertory is all over the map. Of his generation of conductors, none has more consistently defied the stereotypes.

In his concerts here, the most important work has been that landmark of High Romanticism, Strauss' "Ein Heldenleben," but next week in New York he turns his attention to the world premiere of Steve Reich's "The Desert Music," to poems of William Carlos Williams.

"For doing 'Heldenleben', I get some guff from the new music buffs," says Tilson Thomas. "They say, 'Isn't that just some bourgeois trash?' I say no, it has to be approached ardently and faithfully on its own terms. And some of the 'Ein Heldenleben' people say, 'What's this Steve Reich stuff? Isn't that avant-garde?' I say no.

"That's the story of my life. I can't tell you how much. And I wouldn't have it any other way."

Tilson Thomas' high visibility from the beginning, somewhat in the mold of his friend and admirer Leonard Bernstein, has been at times a great advantage, as in his recording career. His records range from the current Beethoven, Tchaikovsky and Ives cycles to a disc of songs he made with Sarah Vaughan.

Assistant conductor of the Boston Symphony at the time he stepped in for Steinberg, he became its associate conductor in 1970 and a principal guest in 1972. In 1971 he was named music director of the Buffalo Philharmonic, from which he departed in 1979 for guest conducting.

He is now principal guest with the Los Angeles Philharmonic and has taken over a new venture there, the Los Angeles Philharmonic Institute, a program for young players and conductors.

"That has turned out to be a thrilling and idealistic experience," he says. "I can't tell you what it is like. Students come from all over the country, all over the world. You learn from it as well. They ask questions, and those questions lead you into further study.

"Because of the students and because there was more time there, I finally got to work out a performance of Bartok's Concerto for Orchestra that I really like. It was based on the phrase structure and not on meter changes. Most of the time in that piece, everyone is so happy just to get through the intricacies of the shifting meters and make a few loud entrances here and there. But no one deals with this wonderful little subtle design of three-bar dance phrases that he uses, and that's the kind of problem I get to work on there."

Tilson Thomas also welcomes the Institute as a personal foil to what he calls "international fast-lane music-making."

"It's a great balance," he explains, "to the excitement and the vertiginous life of a jet-set guest conductor, which is what I am. I am one of the leading jet-set guest conductors . . . not that I'm going to do it forever. I have only been doing it four or five years. And it's very exciting to do this great repertory with great orchestras that have basically very different traditions."

His favorites?

"Well, a number. Of course, my work in Los Angeles and in Chicago, which has been continuosly expanding. And now in Boston again. And with the Philharmonia in London and the Berlin Philharmonic and the Amsterdam Concertgebouw."

A native of Los Angeles, Tilson Thomas comes from a distinguished Yiddish theater family, the Tomashefskys, who were close to the Gershwins. Today he is particularly excited about a new recording of Gershwin music he has made, including some piano pieces that will be heard for the first time (the conductor is also a fine pianist).

"I hate to be egocentric," he declares, "but I really do know how this music is to be played. My father and my uncles all knew George, and I grew up with all these tunesmiths, tune pluggers and Broadway people around me, and I just know how to play it . . . And some of these new pieces, some that Ira Gershwin gave me, are really very eloquent."

There also was a very different side to his musical roots, in Los Angeles and elsewhere. "One of the very nice things in my life was that I came about at just the right time in musical history, so that I came in contact with the older generations, like Piatigorsky, Heifetz, Rubinstein, Stravinsky, these kinds of people."

He played for Piatigorsky's master classes, performed Beethoven for Stravinsky at the famed Los Angeles Monday Evening Concerts and prepared the orchestra for the Heifetz-Piatigorsky concerts.

"They were a strong influence on me in many ways, and all of them predicted in one way or another the course of my life rather accurately."

He recalls some luncheon advice from Rubinstein:

"He said, 'Well, young man, you're an original, you have original ideas. Some of the ideas are wonderfully clear now, and others you are still working with. But keep working, and keep to your vision . . . In my case, I have always played the music of Chopin, but for the first 50 years that I played Chopin everybody told me that it was wrong. Then suddenly they began saying about my Chopin, "Oh, this is the way it is supposed to go." '

The conductor adds, "I can't tell you how many times I have thought of that over the years. He said, 'People will always be confused by you. Just as I confused them, you'll confuse them.' "

Tilson Thomas, who will be 40 in December, is deliberately taking his time. "But I think everything is starting to link," he says.

What is he searching for?

"Well, it's exciting to see your name on a poster at some place that is one of the great institutions of the world. But it's even more exciting when the music is truly great and you realize that you have made a step forward in those rehearsals and performances in your understanding of the piece. That's what I am searching for, a place where I feel the community and the management and everyone else involved wants that to be the kind of musical atmosphere that exists in their city. Is that hopelessly idealistic?"