On the screen, Julia Migenes-Johnson is a child of instinct, a force of nature, a criminal and a witch. "If I love you, watch out," she warns everyone in sight but particularly Placido Domingo -- and her warning is deadly accurate.

In a word, Julia Migenes-Johnson is Carmen: "Bizet's Carmen," as the movie title puts it, to avoid confusion with all the other Carmens currently selling tickets.

Off the screen, Migenes-Johnson is a suburban housewife in her late thirties who has been kept awake for four straight nights by sick children.

She lives on Long Island with her husband, Jervis Johnson, two daughters -- Jessica, 3, and Martina, 10 -- and a dog named Blanco. "I hyphenate my name," she says, "because I would rather have people call my husband 'Mr. Migenes-Johnson' than 'Mr. Migenes.' I think it's terrible, what happens to the husbands of women with well-known names."

Combining life as a wife and mother and as a star of an epic movie about sex and sudden death can have curious effects. "I have two wonderful helpers at home," she says by way of example. "When the movie opened in New York, they went to see it. And when they came home they were saying, 'Now I know what he sees in her.' "

"Carmen," her first movie, is not the first time she has been featured as a sexual presence. Two previous opera roles led naturally to it -- Lulu, Alban Berg's decadent, destructive sex goddess, and Salome, the Richard Strauss femme fatale.

"What I like about opera," she says, "is not just the music, which is beautiful of course, but the personalities. I love all that 'Tosca' stuff -- stabbing and cursing and jumping off high buildings."

She was singing in "Salome" in Switzerland, under the direction of Maurice Be'jart, when word reached her that she was wanted for the "Carmen" movie. "You might say I was slightly stunned," she recalled recently during lunch at the Watergate. She was recognizably the woman who had tormented Domingo in the movie -- not beautiful in the plastic Miss America style but intense, direct, with a no-nonsense air. She was paricularly interested in bread. "Do you have any dark bread, rye or pumpernickel?" she asked a waiter, who ran off to get her a selection. "It's so hard to get good bread in America," she said. After the meal, she popped down a dozen vitamin pills -- "only natural vitamins," she insisted; "I don't take chemicals."

Her career was changed, she said, during a meal in Paris while she was in Switzerland. Be'jart had dinner with Patrice Ledoux, producer of "Bizet's Carmen" for Gaumont Films, and Ledoux complained that his company had auditioned hundreds of singers but could not find a Carmen. "Come home with me and see this girl on my video," Be'jart answered.

Learning the role of Carmen required some mental readjustment because, although she liked the opera, Migenes-Johnson had never imagined herself in that role.

"I had to look at it as a new thing, something involving me. Suddenly I had to zero in on minute details. How could I handle those heavy tones with my light voice? How could I make it believable? But after listening to a couple of recordings, I felt, 'I know where she's going. She's not too far from a personality within me that I can draw on.'

"The other part I got from watching Gypsy flamenco dancers, who are all Carmen. There is a very specific kind of feminine sensuality in these dancers. There is a tension -- energy held in by convention -- and when that energy is unleashed, all hell breaks loose."

Migenes-Johnson is an American who speaks German fluently; she has been singing in Vienna and other German-speaking cities for more than 10 years.

The "Migenes" part of her name is pronounced like the Irish "McGuinness"; it acquired its Spanish spelling after an Irish ancestor moved to South America in the last century. Although her family is Puerto Rican, she never learned Spanish and regrets it.

She is the daughter of a Greek father whose name (Mouziakis) she does not use. She calls her father "the Greek."

"He was the man next door" on Manhattan's East Side, she explains. "My mother had two children who looked like her husband and three who looked like the man next door. There were some very interesting tensions in our house.

"She was married to a cousin and it never really worked. I was 10 years old when my father married my mother, and he never adopted us. His attitude was: Why should I adopt them? They are my children.

"My mother really loved the Greek. They really loved one another. When she died, he died three weeks later. He hardly ever spoke any English; I never understood a word he said. But they managed to communicate."

For "Carmen," she commuted from New York to Paris for nine months, preparing for a role she has never sung on the stage and probably never will. Her voice is not naturally big enough or low enough to fill an opera house with Carmen's rich chest tones. She worked on it for a year to prepare for the movie, and during that time she avoided roles that used the lighter tone natural to her.

Looking back, it seems more likely that she would have ended up on Broadway. At age 6, she was touring with "South Pacific." She was a student at New York's High School of Music and Art, she recalls, and "I wanted to dance, but I was too small and too round.

"One of my teachers wanted me to sing; she said I had a feel for classical music that was rare. I just thought it was pretty. But now, watching myself at that age on tape, I can see that I had a built-in technique. In my teens, I would do things naturally; now I would have to work on it, think about it."

She can watch her teen-age self on videotape because she was chosen by Leonard Bernstein for a television production of Aaron Copland's opera "The Second Hurricane" when she was a junior in high school.

Later, in 1964, she played Maria in a revival of "West Side Story," and from there she went on to play one of the daughters in "Fiddler on the Roof." When Gian Carlo Menotti needed a young singer for "The Saint of Bleecker Street" and then for "The Consul," her career suddenly shifted from Broadway to opera. Almost before she knew it, she was singing "Madama Butterfly," going to Europe for more vocal training and becoming a headliner at the Vienna Volksoper.

After establishing a reputation in European opera and television, Migenes-Johnson began to build an American career in 1979, singing in "La Bohe me," "Mahagonny" and "I Pagliacci" at the San Francisco Opera and the Met. Her exposure in this country escalated in 1980, when she substituted for Teresa Stratas at the last minute in the Met's televised production of "Lulu."

With "Carmen" on her re'sume', Migenes-Johnson seems likely to try for a new career as a movie actress. "Gaumont has signed me for options on four films," she says, "one opera, two with music and one with straight acting."

Despite her stage successes, she says, "opera is basically a hobby. If a role appeals to me, I may sing in three or four productions a year. Other roles I will sing if there is a lot of money and the right kind of conductor -- one like Richard Strauss. You know, Strauss said, 'I never look at the brass and percussion; it only encourages them.' That's the kind of conductor I like to work with."

Most of her operatic ambitions have been fulfilled, but she still has one fantasy. "Some day," she says, "I will go to Bayreuth, get myself a costume and a spear and bribe the first Valkyrie to let me take her place. I just want to know what it feels like to sing 'Hojotoho' on that stage."

Meanwhile, she says, the road to operatic fame in America seems to lie through typecasting: "Whores, criminals, Gypsies and servants."