NEW YORK — 'IT'S a film based on the play, not a film of the play."With that caveat out of the way, Terrence McNally begins to relate how he transformed his highly-acclaimed off-Broadway hit "Frankie and Johnny in the Clair de Lune," into a big-budget Paramount Pictures feature film "Frankie and Johnny." Taking a writing break in his New York apartment, Mr. McNally explains that the story is about how "a lot of people are not living in relationships any more, and seem to be making a life for themselves on their own." The play traces the rocky early stages of a romance between an ex-convict short-order cook and a disheartened, tough-acting waitress who work together in a Manhattan diner. The entire play occurs during one night in her cramped Hell's Kitchen tenement apartment. "I wrote the play for Kathy," he notes, referring to Kathy Bates, who performed the role for nearly a year. Speculation was stirred by the casting of Michelle Pfeiffer in the lead. Critics said that the story's essence, built so heavily on the personality of a woman not considered attractive, would be compromised. McNally responds, "I did the film script first, before anyone was cast." McNally says that Ms. Pfeiffer is "a real character actress, who's different in every role she takes. This is a very naked, brave performance," revealing vulnerabilities in the character and the actress. "There were no changes made," he says, once Pfeiffer was cast. McNally says he decided that he was "not so much adapting the play as retelling it. I put the play in a bottom drawer, and locked it." To expand the story, he introduced the owner and co-workers at the diner, as well as Frankie's neighbors and family, but retained the basic two-character focus. McNally admits he might have had some concern about taking this small tale and giving it the big-screen treatment, had he not seen an earlier film by director Garry Marshall, "The Flamingo Kid.It reminded me, in tone, of my play," he notes. "Some movies about blue-collar people romanticize their lives," but that film presented them honestly, a reflection, McNally says, of the fact that Mr. Marshall "is really a working-class kid from the Bronx." Marshall conducted sit-down readings of the scripts with the actors, after which "the script underwent minimal changes," focusing more on clarifying points than changes of emphasis. "I pretty much write on instinct," he continues, explaining that he did not story board the screenplay scene by scene before he began. Instead, he sat down and wrote, letting the story, the characters and the emotions express themselves, which is how he writes plays. "I listen to music a lot, and think I have a good sense of structure, when to repeat something, when to go back or move on." His current play, "Lips Together, Teeth Apart," running at the Manhattan Theatre Club, about two couples sharing a b each-house weekend, represents his skill at dramatizing the small, random moments in people's lives. "I don't think of life as well-plotted!" he says. McNally welcomed some of the improvisational elements in the film, such as a moment when Johnny takes out a small rubber ball and bowls with it. "Al just brought that with him. I've always encouraged actors or a director to find behavior. I don't consider it competition." McNally, who found Alfred Uhry's screenplay for Mr. Uhry's play "Driving Miss Daisy" a "wonderful adaptation," has written several highly-regarded plays, including "The Lisbon Traviata," and "Bad Habits." He did adapt his play "The Ritz" for the screen 15 years ago, "where we essentially did the play in front of a camera." His 1989 script for "Andre's Mother," on PBS's American Playhouse, won an Emmy. This time, "maybe I was still so naive and green that I was fearless. But I decided I was not going to t ry to 'save' dialogue. If something didn't fit, out it went.... This is essentially a small story, and once you go beyond two hours, it's just pretentious." How does he feel about a film that has elements different from the play? "The film is very true to the feeling of the play. The play's not going to go anywhere. You can always buy it in a bookstore. And hopefully, a theater will always be doing it, somewhere."