STEVEN WINN, CHRONICLE STAFF WRITER

TO ENTHUSIASTIC advocates and more skeptical observers alike, Terrence McNally's ''The Lisbon Traviata'' was one of the wittiest, most audacious and most unsettling shows of last year's New York theater season. After a 4-month off-Broadway run that ended abruptly in January, McNally's startling drama opens this week at the Marines Memorial Theatre for its first engagement outside New York.

Along with its uniformly admired strengths -- most notably a bravura tragicomic performance by Nathan Lane as the lonely homosexual opera fanatic Mendy -- ''Lisbon'' holds more controversial aspects.

Primed by Act One for an evening of tart, bittersweet banter about subjects ranging from pirated Maria Callas recordings to gay dating styles in AIDS-era New York, the audience returns after intermission to a radically altered environment. The change is signaled first by Philipp Jung's set design, which replaces the cozy, quasi-Victorian cocoon of Mendy's apartment with the austere chrome-and-leather severity of his gay friend Stephen's dwelling. In a deadly serious second act that relentlessly exposes the sterility and decay of Stephen's emotional life, ''Lisbon'' spins around 180 degrees from its initial comic leanings and heads inexorably for a tragic conclusion.

With grand opera as both its ostensible subject and its stylistic model, ''Lisbon'' is, by design, a work predicated on expressive extremes. If this is a play about highly sophisticated and self-conscious people of the late 20th century for whom opera happens to be an obsessive hobby, it is also, as director John Tillinger puts it, ''about passion itself,'' which, he maintains, people find ''jarring and unnerving in a modern vernacular.''

As for ''Lisbon's'' unorthodox dramatic conventions, McNally explains, ''I set out, quite consciously, to write an opera buffa in the first act and a verismo opera in the second.''

To some audience members, perhaps most tellingly to the New York Times critic, McNally was only too successful in contrasting ''Lisbon's'' two halves.

Where, went the standard line of questioning, was the preparation in the effusive Act One for the caustic burn of Act Two? Why was the captivating Lane banished from the stage entirely after intermission, his presence confined to the telephone answering machine in Stephen's apartment? And why, above all, did the play end with an act of violence that seemed at once inadequately motivated and extravagantly premeditated?

As Tillinger remembers the show's New York run, which began at the midtown Manhattan Theater Club and continued at the uptown Promenade Theater, ''People either bought the ending completely, or they switched off and denied the truthfulness of the entire evening as a result. That was a phenomenon we just had to address.''

Last fall, McNally, author of ''The Ritz,'' ''Frankie and Johnny in the Clair de Lune,'' the book for the recent musical ''The Kiss of the Spider Woman'' and many other works, substituted a new, less sensational ending for ''Lisbon.'' The revised script won Times critic Mel Gussow's acknowledgment that the denouement and the play itself were now ''more moving,'' though he added that he remained troubled by ''basic problems of structure and motivation.''

Tillinger was more concerned about ''the sorry state of theater in New York'' when describing the failure of any commercial producer to option ''Lisbon'' after one poor week at the box office brought the Promenade run to a sudden close. ''It's difficult for me to understand how (Broadway producer) Robert Whitehead would say this was one of the most exciting experiences he'd had in the theater in 10 years,'' says Tillinger, ''and then we never heard another word from him.''

''The Lisbon Traviata,'' as anyone can attest who has been a part of it or seen it, is a play that gets under the skin and won't go away. ''People -- often gay people, interestingly enough -- can be quite vituperative about this play,'' says Tillinger. ''A lot of women, on the other hand, identify very strongly with Mendy and Stephen. At one time or another, as they put it, they've been both of those men.''

Richard Thomas, the well-known stage and screen actor who will make his San Francisco debut in the role of Stephen, saw the show in New York and ''knew immediately this was a part I wanted to do.'' Far from being daunted by a character who remains largely muted in the first act before unraveling in the second, Thomas saw a role that was ''very much like real life.''

As for Stephen's apparently radical emotional turnaround in the middle of the play, Thomas argues, ''That's often how life is, isn't it? You go out and have a wonderful evening with people at dinner and then come home and discover your dog has died.'' The character's ''agile mind'' and ''lightning transitions,'' Thomas believes, provide the key to hinting at the darkness of Act Two without giving it away in the first act.

''The idea that Stephen is in crisis needs to be established in Act One,'' the actor says. ''But you can let that emerge slowly. There's lots of time to play poker, and you can let things out in flashes and then conceal again. This is a man who can be terribly witty in the midst of horrendous circumstances. At the same time, he's living absolutely in the sway of his own passions. That's all very appealing to me.''

For Lane, who will reprise the virtuoso role he created in New York, playing Mendy again in California offers at least two appealing bonuses -- a chance to come to San Francisco for the first time and an opportunity to showcase his talents in Hollywood.

Lane, who can show some of Mendy's dishy, self-deprecating humor offstage, is forthright in his appreciation of the part McNally wrote. ''Mendy is drawn on a very large scale, like Shakespeare -- or a Restoration character, as one critic said. In many ways he's very easy to play because his need is so great. The trick is to play the flamboyance and the outrageousness without going over into caricature.''

While Lane won't reveal whether Mendy gets to appear after intermission in the ''Lisbon'' script McNally has revised anew for the West Coast, the actor is frank about his preference for the original ending of the play. ''The new ending lets people off the hook a little. Of course,'' he adds, ''I'm not the one who had to play it.''

At the close of a day's rehearsal at the Mark Taper Forum, where ''Lisbon'' is being readied for its San Francisco run and a subsequent engagement in Los Angeles, McNally is circumspect, almost defensive about the changes in the play. They aren't so much changes, he contends, as ''a process'' that happens in the course of any play's development. None of it, he insists, is a capitulation to the press or to anyone else.

McNally and Tillinger agree that the two acts of ''Lisbon,'' different as they are, needed to be integrated more completely to work for an audience. The playwright has rewritten about 10 to 15 percent of both acts to that end, while Tillinger has been concentrating on ''the humor, the vulnerability and the sexuality of Stephen. Humor often hides pain,'' Tillinger says. ''And that's a way to prepare you for the seriousness of what's to come.''

Unchanged are the play's musical themes that first inspired McNally, a lifetime opera lover. ''I wanted to write about the impact a performer as persuasive and unique as Callas can have,'' he explains, revealing his own fascination with the controversial star, whom he saw in her Metropolitan Opera debut. The play's title -- and the preoccupation of the characters in the first act -- refers to a pirated recording of a 1958 performance of Verdi's ''La Traviata.'' ''I could have called it 'The Genoa Tristan' or 'The Chicago Trovatore,' '' McNally says, mentioning other fabled and elusive Callas performances. ''This is a play about the unattainable.''

In McNally's first play, ''And Things Go Bump in the Night,'' the main character was a retired diva. Ten years ago he began a script, which he still intends to finish someday, about Bellini and ''I Puritani.'' For now, ''The Lisbon Traviata'' is McNally's daring fusion of opera's heightened reality and a naturalistic, contemporary stage idiom.

McNally's next play, slated for a spring production at the Manhattan Theater Club, is ''Lips Together, Teeth Apart.'' Written specifically for Lane, Anthony Heald (''Lisbon's'' Stephen in New York), Kathy Bates (co-star of the original ''Frankie and Johnny'') and Christine Baranski, it's ''about friendship.''

As for Callas, McNally figures he's probably had his say for now. But there is, he reveals, a singer he likes ''even more'' -- Edith Piaf. ''Something about those unconventional voices,'' he muses. ''But you know, I never understood all the controversy about Callas' supposedly harsh sound. To me, from the moment I first heard her, it was always a beautiful voice.''

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