Ayoung waitress is heading for a brick-wall reality check. She's making small talk with another young woman who's there for lunch; a string of chirpy "me toos" has gotten her as far as she's going to get. You're a musician? Me too. Here in Boston? Me too. Where are you studying?

This one is going to be painful.

"Actually, I play with the BSO," says Nurit Bar-Josef, who admits this kind of thing happens from time to time.

Bar-Josef is a polite, low-key woman who is not just any musician in the Boston Symphony Orchestra but, until this month, was the assistant concertmaster. She's only 26, so it's easy to see how a young waitress would assume that because they're contemporaries, she and Bar-Josef might also be classmates.

But no. Bar-Josef may look as if she bicycles to work at a local coffee bar, but she is one of the most accomplished musicians in a city filled with great players.

Beginning with the opening-night concert on Sept. 19, Bar-Josef will be the new concertmaster of the National Symphony Orchestra. She is one of the youngest players to hold such a prestigious position at a major American orchestra. But she is joining a troubled orchestra that has been uneven in recent years, a group that has just ditched its president and is looking for a new general manager. At a time when the NSO needs leadership, Bar-Josef is quietly taking a job with a lot of behind-the-scenes influence over the direction of the ensemble.

After almost 20 years, the most significant musical position in the NSO -- save that of the conductor -- came open at the end of last season with the retirement of concertmaster William Steck. The final years of Steck's tenure, and his departure -- sources say it was not amicable -- weren't easy for either the orchestra or Music Director Leonard Slatkin. Both Steck and Slatkin were considered "difficult" personalities, often remote, incommunicative and unapproachable; the combination, and lack of rapport between the two, was bad for morale.

Steck's retirement came and went without celebration -- no parties or public farewells. With no explanation last spring, a flier inserted in NSO program books indicated that the orchestra was auditioning a new concertmaster. Longtime observers of the orchestra considered it a shabby way to say goodbye to a veteran player; insiders said it was long overdue.

Based on a single performance each, it was difficult to say much about the three outside candidates, though the fourth, the NSO's own Elisabeth Adkins, was well known and well liked by the orchestra. The initial impression made by Bar-Josef was just that -- an impression and by no means definitive -- but it was a positive one.

"Bar-Josef was showing her potential to the NSO, and in turn, the NSO reflected its best back at her," this critic wrote at the time. "It was an exciting concert, more accurate, more musically engaged, more expressive than the norm. The orchestra, sometimes uneven, was playing in top form."

By the end of the season, it was clear that the orchestra, and Slatkin in particular, were leaning heavily toward Bar-Josef. Although there was disappointment among some of the players that Adkins hadn't received the position -- and concern that because she was pregnant at the time, she was under special pressure -- the orchestra went out of its way to suggest it had been a close race. "Having Elisabeth and Nurit on the first stand would be any music director's dream," said Slatkin.

A lot is riding on Bar-Josef's skills as a musician and, in a quiet way, as a politician. The concertmaster is head of an orchestra's string section, a liaison between the conductor and musicians and, in many orchestras, a de facto personnel director. Although the NSO has undergone significant personnel changes since Slatkin's appointment was announced in 1994 -- about 25 percent of the orchestra has arrived in the last seven years -- the arrival of Bar-Josef is the single most influential change Slatkin can make.

He has chosen a very young woman, but by no means someone new to the sorts of responsibilities the job requires. Bar-Josef landed in the orchestra world early -- she was only 21 when she got her first job -- and has quickly moved into positions of ever greater responsibility. She is a graduate of the Curtis Institute of Music and studied at the Juilliard School, though she left Juilliard early to take up orchestral work. There was a stint, in 1997-98, as principal second violin with the St. Louis Symphony (after Slatkin, a former music director in St. Louis, had left), and then it was on to Boston.

"The age thing never really crosses my mind," says Bar-Josef. "I've been sitting assistant here, watching the concertmaster, and once in a while I even sit in that chair. It always feels pretty natural to sit there. I know that the responsibilities are big, but I feel I'm ready for it."

In person, Bar-Josef is natural rather than polished. On an unseasonably hot day, she's running around town. It's the day after the NSO's announcement of her appointment, and she's hard-pressed to squeeze in a lunch between a doctor's appointment in the suburbs and the regular obligation to play a Boston Pops concert that evening.

With the NSO job, her life suddenly became very complicated; she had to find an apartment in a city she doesn't know, play the BSO's summer season at Tanglewood in the Berkshires, and at the end of August go on tour to Europe one final time with her old orchestra. She doesn't seem flustered.

"I'm well aware of what this job entails," says Bar-Josef. "In Boston I spent a lot of time with the board members of the orchestra, doing fundraisers, board events, community programs. It has never been just a 'play your music and go home' sort of job."

As one of the "lieutenants" in Boston, she enjoyed a minor, niche kind of celebrity.

"A lot of audience members in Boston will come up to the stage and greet you," she says. "It becomes a normal thing. We also do a lot of prelude concerts where you're in much more contact with the audience. It happens, people recognize me, a couple of times in the gym, at the local supermarket. 'Hey, aren't you in the violin section?' "

In Boston she was playing in one of the finest halls in the country, with an orchestra that, despite critical discussion of a recent decline, has one of the most distinguished, burnished string sounds in the country. It is also an orchestra with a strong claim on the home town's attention span; the Boston Symphony Orchestra, and perhaps even more so the Boston Pops, are civic institutions in a way that the NSO isn't.

As music directors pursue international gigs and bigger salaries at the expense of concerted time with any one single orchestra, the expectations on concertmasters have risen. Major American conductors often spend only about three months of the year performing with their orchestras; concertmasters, by contrast, are on duty year-round, a regular presence with both musical colleagues and the audience. They are by definition musically accomplished; if they are also young and photogenic, there is the added possibility that they can become public faces for their orchestras.

"I'm not a huge public speaker," says Bar-Josef. "Maybe I'll come around to being one once I'm in a political town. Put a violin in my hands and I have no problem communicating with an audience, but if I'm available, I'll be ready and happy to do other kinds of things."

The NSO has said it has no particular plans to push Bar-Josef into that role. But it will be a strong temptation.

"I understand that it helps the public when they see not just a concertmaster, or musician, but that you're a human being," she says. "It takes down a barrier."

Whether Bar-Josef emerges as a public figure depends, ultimately, on the fortunes of the orchestra. Former NSO music director Mstislav Rostropovich, one of the world's great cellists, helped the orchestra build a formidable cello section, but while the tenure of a great Russian dissident, especially in the waning days of the Cold War, gave the orchestra a certain glamour, the lingering consensus is that the group suffered technically -- especially in the overall string sound.

No other section of an orchestra is as difficult to fix as the strings. Hiring a new principal horn player or a new principal flute can work overnight wonders, but changing the string sound must be done simultaneously from the ground up -- by hiring new players and bringing cohesion to the old ones -- and the top down -- by changing the section's leadership. Union contracts, and basic human decency, prevent American conductors from cleaning house and hiring a whole new team, so change comes slowly, incrementally.

The chemistry between Bar-Josef and the other players will be critical. Steck was considered old school; one young player remembers his first contact with him as blunt and discouraging. Bar-Josef, by contrast, emphasizes her warm feelings for the players.

"I arrived for the audition just as the orchestra was getting back from a tour," she says of her audition week last March. "I thought they might be tired, or cranky, but they were wonderful to work with. They are very friendly, like a big family. They were very encouraging."

If she hasn't read books on "open" management styles, she's divined their spirit.

"I would like to feel close to my colleagues," she says. "I'd like them to be able to ask about anything. I don't want to be one of those concertmasters that say, 'I'm your leader, this is what I want.' "

Underscoring the importance placed on her are the NSO's attempts to find Bar-Josef another violin. She currently plays on an early 20th-century instrument, a 1908 violin made by Fagnola, a distinguished craftsman from Turin, Italy. Fagnolas are highly respectable instruments and can fetch $50,000 to $75,000 in auction, but the NSO is looking to put something even better in her hands.

"My violin is a fine instrument, but it's not a grand instrument," says Bar-Josef. "They've been looking, and it's still in the works."

An NSO spokeswoman says the question could be resolved early in the season. It's a matter of finding an instrument and money, or an indefinite loan from a generous collector.

"A different instrument changes the sound," says Bar-Josef. "If you have a powerful instrument that can be heard, it really helps. I think for the people around you, and for the other principals, it's really nice to have something that can fill up the hall. It's also a good thing for the orchestra -- and it's good press."

As a player, Bar-Josef emphasizes focus and concentration.

"It always depends on the venue, and what you're playing, but you do get nervous to varying degrees," she says. "I find that if I'm relaxed, then something's wrong. It's almost better to have that extra edge."

Otherwise, she's cautious about the musical style she'll bring to the orchestra.

"I can't honestly say I have a favorite player [among today's violinists], though my absolute favorite violinist is Fritz Kreisler," she says of the great early 20th-century romantic master. "I always feel I was born at the wrong time. I tend to go more for the old-style-type playing, but it depends on the orchestra. When players with that style came into Boston, there was a deep level of respect. When Ida Haendel came in, it really brought the whole level of the playing up."

Haendel, a Polish-born, English-naturalized player, epitomizes a gracious yet wild style of playing that is extremely rare today. If Bar-Josef leads by example, and takes Haendel as an example, there's real hope for the NSO's string sound.

As for Bar-Josef's own sound, there will be at least one major opportunity this season to hear it on display, when she appears as the soloist in Prokofiev's Violin Concerto No. 2 with the NSO in February.

"I'll do a little solo work, but it's not a huge part of my life," she says, though she hopes to make chamber music a major part of her extracurricular activities.

"My love has always been for chamber music, from early on," she says, remembering when her parents took her, an only child, more often to chamber and solo recitals than to orchestra performances -- in Boston, no less, where the BSO rules. "Once I'm settled in, when I'm familiar with the job, I'll start doing that. I can't see my life without chamber music."

That, more than anything, may define her as a musician. Orchestras tend to be authoritarian institutions, managed by bureaucrats and conducted by autocrats. Chamber music, on the other hand, is collegial, spontaneous and, when done well, far more than sum of its parts. The best orchestras play like massive chamber ensembles; if Bar-Josef helps to bring a chamber sound to the NSO strings, the orchestra could be transformed beyond recognition.

"I know that the responsibilities are big, but I feel I'm ready," says 26-year-old Nurit Bar-Josef, who debuts as the NSO's concertmaster Sept. 19."They are very friendly, like a big family," says Nurit Bar-Josef of her new colleagues at the NSO.