'Six Characters' finds a home; Mozart's Mass; 'Romeo and Juliet'

''Born alive, they wished to live.'' So says Luigi Pirandello about the sad family of characters which appeared to him so vividly that he wrote a play for them: ''Six Characters in Search of An Author.''

And live they do - at the American Repertory Theatre (ART). Director Robert Brustein has brought fresh vigor to Pirandello's saga of characters who interrupt a rehearsal to play out their destiny. It's a splendid production. He has pared down extraneous characters, jazzed up the translation, and placed it in the Loeb Drama Center - today. It's so current that one actor asks another if he's going to the opening of ''42nd Street.'' Mr. Brustein's premise is that Pirandello wanted his audience to feel they were at a real rehearsal, not a play about a rehearsal, so the playwright used a real play - one of his own. Brustein's Actors rehearse ART's own ''Sganarelle.'' With the house lights still on, the actors stroll onstage, chat, warm up. The dividing line between ''real life'' and ''the play'' is erased. When the Characters, dressed in mourning, suddenly appear the effect is electrifying.

Pirandello tackles the notion that there is any such thing as a fixed truth. Are the Characters, given life by their author and then abandoned, any less real than the Actors, who spend their time trying to make written characters live and breathe onstage?

The playwright says no. The Characters have raw urgency and ceaseless grief, while the Actors have the confidence of baby-boomers. The lack of communication between them is fuel for both humor and frustration.

Linda Lavin (TV's ''Alice''), as the Mother, is a frail, broken bird, a fountain of tears. Lise Hildboldt, the Stepdaughter, spews out a molten torrent of contempt. Robert Stattel, as the Father, pleads with great intensity for his family's integrity as living characters. The others are equally splendid, the Actors providing a perfect foil to the distraught family.

The chatter about going to the Olympics Arts Festival is, however, self-promotional. The family members are from different eras: The Mother has the halting English and clothes of an old-country matron and the Stepdaughter is decked out in a flouncy miniskirt.

You can live with such things, though. A show that combines stunning but unobtrusive technical effects and gut-wrenching performances is a rare bird. And this one flies away June 3.