What do Leonard Bernstein, Helen Hayes, Otto Preminger, Carol Channing, Truman Capote and George Balanchine have in common? All are giants in the performing arts. And all are -- or have been -- clients of Arnold Weissberger, one of the world's foremost theatrical attorneys. Now in his 50th year of practice, the Brooklyn-born, Westside-raised Weissberger has been representing stars ever since a chance encounter brought Orson Welles to his office in 1936. "Most of my clients are involved in making contracts that have to do with plays or films or television," says Weissberger on a recent afternoon. The scene is his small, richly furnished law firm in the East 50s. Dressed in a dark suit, with a white carnation in his buttonhole to match his white mustache, Weissberger looks very much like the stereotype of a business tycoon. "Part of my job," he continues, "is to be familiar with the rules of guilds and unions. And I have to know about the treaties between countries that affect the payment of taxes." Smiling benevolently, his hands folded in front of him, the gentlemanly lawyer quickly proves himself a gifted storyteller. In his upper-class Boston accent, acquired during seven years at Harvard, he delights in telling anecdotes about his favorite performers. Not shy about dropping names, Weissberger drops only the biggest, such as Sir Laurence Olivier -- a client who had invited him to lunch the previous day -- and Martha Graham. His work is so crowded that whenever he has to read anything that is longer than three pages, he puts it in his weekend bag. Yet Weissberger devotes an hour or two every day to one of several philanthropic organizations. At the top of his list is the Martha Graham Center of Contemporary Dance, of which he is co-chairman. "I consider her one of the three great seminal figures in the arts in the 20th century, and I prize her friendship enormously." The other two outstanding artistic figures of the century, he says, are "Stravinsky, who it was also my privilege to represent, and Picasso, who I did not represent." He serves as chairman of the New Dramatists, a group that nurtures young playwrights; he is a board member of Fountain House, a halfway house for ex-mental patients; and he is chairman of the Theatre and Music Collection of the Museum of the City of New York. On Monday through Thursday, Weissberger lives in a luxurious Eastside apartment that he shares with his longtime friend, theatrical agent Milton Goldman. Each Friday after work, Weissberger departs for Seacliff, Long Island, where he owns a house overlooking the ocean. Goldman and Weissberger, whose careers have run a parallel course during the 35 years of their acquaintance, travel widely each summer, generally spending a month in London, where both have many clients.