Mstislav Rostropovich loves to tease. Even by his standards, though, he carried it to extremes in March last year when he hired the National Symphony's new concertmaster, William Steck.

It was one of those slightly uncomfortable cases in which the NSO decided it needed a new leader--as they call concertmasters in most countries--long before it was close to finding a replacement. Deadlines were approaching. Thirty or 40 violinists had auditioned for the job, unsuccessfully. Steck, who had been Robert Shaw's concertmaster with the Atlanta Symphony for eight years and was not eager to leave there, suddenly entered the picture on the initiative of some old friends who play in the National Symphony. A decision had to be made. And National Symphony Executive Director Henry Fogel, who had been enormously impressed by Steck, decided to fly him to Paris to audition for Rostropovich.

"He was rehearsing a French orchestra in the Salle Pleyel," recalled Steck, 49, a featured soloist in the concerts of the Bach Brandenburg Concertos last night and tonight at the Kennedy Center. "Slava was cordial. I was invited into a little room. And for about 30 minutes he asked me to play a variety of solo things. There was a little Bach. He wanted a portion of the Brahms Concerto. There were parts of the solos from Strauss' 'Ein Heldenleben' and Rimsky-Korsakov's 'Scheherazade' and those solo passages from the Brahms First and the Shostakovich Fifth.

"Then he suggested going across the street to a restaurant. But before we went out, he said he had to make a phone call. So we just sat there Steck's wife Ann, who is also a violinist, was along . But he came right back, saying that all the phones in the hall were backed up. So we went on to the restaurant. He tried his call again, and when he came back . . . he said, 'I have told the office all they need to know.' The dinner went on. And there was not a word about whether he liked me or didn't like me. I didn't know what was happening.

"Then we went back to the Salle Pleyel. He stopped to talk to the hall manager. And suddenly he turned and said to the man, 'By the way, I'd like you to meet William Steck, who is the new concertmaster of the National Symphony Orchestra.' And that's how I found out."

And that's how the National Symphony made its most important hire of a musician since Rostropovich himself. In fact, there is an adage among orchestra players that "conductors may not last, but the people they hire do," especially in this era of strong unions and 52-week orchestra contracts.

And in one short year, it is widely felt, Steck has had a more positive influence on the sound of the National Symphony than anyone in recent years except the music director himself.

Oddly enough for an orchestra conducted by one of the world's most celebrated string players, the strings of the National Symphony--the upper ones, at least--were regarded as a weak spot in an orchestra that otherwise was on the way up artistically. Tone was unpredictable and often erratically focused. In the loud passages, the first violins were sometimes overwhelmed by the good brass and splendid wind sections--a particular problem because the violins carry the main line of a large-scale symphonic work more often than any other section. Rightly or wrongly, much of the blame was placed on Miran Kojian, who had been the concertmaster for more than a decade and is no longer with the orchestra.

Just what does a concertmaster do? "Well," observed Steck, "he obviously comes out alone just before the conductor does and signals the oboe to play an A," by which orchestras tune themselves. But if it is as simple as that, why not make the first oboe the concertmaster?

The concertmaster plays the solo violin passages in symphonic works, but so does the first flute in the flute passages. So why not make the flute player the concertmaster?

The concertmaster's most important job is leadership. The strings are by far the largest section of the orchestra, and they require a great deal more coordination than the rest of the orchestra.

The marking of parts for bowings and other expressive purposes is perhaps the part of the concertmaster's job that most affects the quality of an orchestra. The conductor may change them in rehearsal, but usually doesn't. The idea, said Steck, is for the section "to sound like one enormous violin." It is much easier said than done. If 11 violinists articulate a passage crisply and 11 others do not, the articulation will not be crisp.

One of the reasons Steck was chosen for his post here is his considerable experience as a violinist. For three years, he played with the Philadelphia Orchestra under Eugene Ormandy, in a string section that was the ne plus ultra of tonal refinement. That experience was one reason for the virtuosity of the Ormandy performances on the Rachmaninoff Second Symphony with the National Symphony this spring. "I had played often enough with him that I pretty well knew what he wanted," Steck observed. "I knew details that he didn't even mark in his own scores."

But it was the experience with George Szell at the Cleveland Orchestra that most influenced Steck's notions of phrasing. Steck even went to the trouble of making copies of many of Szell's marked scores and is much influenced by them today in his work with the NSO strings.

"Until I got to Cleveland," where he stayed five years, Steck recalled, "I had been sort of swimming against the current in matters of style and phrasing. When I got there I was absolutely floored by what I heard. It told me that what I had been working at was right. I had been nibbling at a gold mine, and there it was in Cleveland. It was a special sound, one that added up to more than the sum of its parts. As Szell used to say, the idea is 'to sound completely spontaneous, but meticulous at the same time.' "

He went to the piano to try to illustrate his point. First he played the opening theme of Mozart's 40th Symphony in the Szell style, and then in the Ormandy style. The former was phrased in what he described as a "keyboard style," with crisper, more even articulation; Szell was a fine pianist. The latter was smoother, more "operatic," said Steck; Ormandy was a violinist when he was young.

Once Steck was hired, the National Symphony made a conscious decision to demonstrate quickly his quality as a violinist, to the public and to his fellow players. "The best way to lead is by example," Steck noted.

He was a soloist in a concerto during one of last summer's concerts on the West Terrace of the Capitol, and, said NSO Executive Director Fogel, "it was no accident that we opened last season in the fall with 'Scheherazade,' " which has strenuous solo violin parts.

Steck must also play the politician. Fogel observed, "He can talk to people, and he can do it in a nonthreatening and nice way." Asked what convinced him to come here, an action the Atlanta Orchestra strongly opposed, Steck answered, "Slava . . . and the higher quality of this orchestra."

Steck doesn't suggest that the National Symphony strings would yet be mistaken for those of the Berlin Philharmonic. But in addition to clearer articulation and more precise dynamics, there's a new body to the sound, a sharper focus. This has also improved the orchestra's balances, because the violins project better. Rostropovich certainly deserves some of the credit for this, but on Steck falls the main responsibility for week-in, week-out work with the players.

Steck is a quiet, unostentatious man who speaks with a slight stutter. He was born in Wyoming, to parents who were violinists. But none of his three children seems headed that way. The family lives in Alexandria, and his pastimes are golf and model railroading.

Unlike most concertmasters, he does not play a 17th- or 18th-century Italian violin. Instead, he has a 1980 instrument by the craftsman Sergio Peresson, whose creations are played by many members of the Philadelphia Orchestra. "It is rich, easy to play, with a big sound," observed Steck. "And it's within budget range."