Exploring the family business

One place Thomas has not been seen in more than two decades, however, is on the podium in front of the ensemble that played a significant role in the launching of his conducting career: the Boston Symphony Orchestra. After a summer at Tanglewood, where he won the prestigious Koussevitzky prize, Thomas was appointed as an assistant conductor to BSO music director William Steinberg. He was just 23, but quickly began climbing the ranks and seemed destined for an important career. A Globe magazine article in 1972 said as much in its title, "The Inevitable Future of Michael Tilson Thomas."

Weighty comparisons were made to Leonard Bernstein but Thomas set his own course, and for a long stretch in the 1990s, it did not include much guest conducting. He returns this August to Tanglewood, where he will lead the BSO for the first time since 1988. Even he seemed surprised at how long it's been.

"Time flies when you're having fun," Thomas said with a deep belly laugh, speaking by phone from Miami. "I was doing a lot of other things, and I certainly can't say it was particularly the BSO I was not working with. I tend to get very involved in orchestras I'm doing projects with." How was he feeling about returning to where it all began? "I know there are going to be powerful flashbacks to so many very formative events of my early years. The period I shared with the Boston Symphony was a wonderful moment in which I think that my enormous passion for music - the completely unguarded passion that I felt about music as a lad - was very transparent and openly shared with them. But it was inevitable that I had to go and figure out a lot of things about how I wanted to accomplish what I wanted to accomplish."

And that he has done. Since taking up the reins in San Francisco, Thomas has built a forward-looking and widely respected partnership in that city. In recent years, he has also been delving into his own fascinating family history, and has developed an evening-length project he will also be bringing to Tanglewood's Ozawa Hall. Entitled "The Thomashefskys," it explores the extraordinary life and times of his grandparents Boris and Bessie Thomashefsky, two larger-than-life figures who as actors and impresarios, played a central role in the early-20th-century flourishing of the Yiddish theater on New York's Lower East Side.

Some of the most popular Yiddish plays were, in the famous phrase, "translated and improved" versions of the classics, so that "Hamlet" became "Der Yisheva Bokher" ("The Yeshiva Student"), while other popular shows dealt with everyday themes more central to the lives of the audience. Over the years, the Yiddish theater incubated some big Tin Pan Alley talents, and more generally, helped countless immigrants orient themselves to - or for a few hours, escape from - their new lives in America.

Thomas's piece is both a richly conceived tribute to his grandparents and a guided tour of their vanished world, weaving together film and archival materials, the conductor's personal memories, and live performances of a few hit numbers. Thomas, who has been presenting "The Thomashefskys" around the country in recent years, says that delving into family history has helped him gain some valuable perspective on his own approach to classical music and to working with orchestras.

"In my father's line they were [cantors], then village entertainers, and then theater people, and then all this came around to me to be a classical musician," he said. "I think that I tend to recognize certain qualities in the music because of the nature of my family's background. Particularly, in the approach I've taken to Mahler, I hear so much village music, and situations arising from village music, in his big structures."

"I also think," he added, "that because I was raised in a family that came from a theatrical background, I do approach these things more from the perspective of being a director, in that I'm working with great performers [who are like] great actors. I'm trying to clarify for them the space in which they do what they do, and trying to help them be in character, to help them be the character."

Michael Tilson Thomas leads the BSO at Tanglewood on Aug. 14 and 23, and hosts "The Thomashefskys" on Aug. 19 and 20. 617-266-1200, www.tanglewood.org

Jeremy Eichler can be reached at jeichler@globe.com.