Last month, film history was made when a modest thriller, Joseph Ruben's "Sleeping With the Enemy," opened to a $13.7 million weekend - a figure that made it the most popular film ever to open in February as well as the biggest hit a female star has ever been able to spark entirely on her own.

The female star is, of course, Julia Roberts, the 23-year-old little sister of actor Eric Roberts who first came to attention in the unexpected independent hit of 1988, "Mystic Pizza."Since then, she has artfully tossed her long brown mane, flashed her big bright eyes, and unfurled her impossibly wide smile in "Steel Magnolias," "Flatliners" and "Pretty Woman," the last becoming the second-highest grossing film of 1990 largely on the strength of her delighted, startled expression on opening a jewel box.

Even at the peak of their popularity, such major female stars of the recent past as Barbra Streisand, Jane Fonda and Goldie Hawn generated nothing like the warm, widespread affection that Roberts has coaxed from the American public. Roberts has even been nominated for an Oscar for her performance in "Pretty Woman," an acknowledgement almost unheard of for a traditional star turn in an unabashedly commercial film.

As the French critic Yann Tobin has pointed out, there has been nothing like the Roberts phenomenon since 1953, when another coltish young performer, Audrey Hepburn, made her lead debut in "Roman Holiday," and the world fell in love with her. "Upon seeing the film," Tobin writes, "every spectator - man and woman, young and old - succumbed exactly as the camera did to the mobile face, the ravishing smile and the spontaneous charisma of a young actress."

"To succumb" seems exactly the right verb, because Roberts' charm is not one of persuasion or slow insinuation but of immediate conquest. It is, for men, either love at first sight or eternal indifference; for women, a sense of instant identification or profound annoyance. It's clear, however, that for the vast majority of Americans, as we step gingerly into a new decade, Julia Roberts is the woman we want either to be or be with - the defining figure of a new femininity.

At this point in her career, Roberts is not a great actress. She has an extremely limited range of expression - beaming delight on the one hand, trembling anxiety on the other - and practically no depth of personality to back up her small effects. Next to an Anjelica Huston or a Debra Winger, she barely seems to exist.

But performing ability and stardom have never been closely related (though it's nice when it happens). We pick our movie stars on the basis of what we can immediately read in them - in their faces, voices, bearings and bodies - rather than on the characters they self-consciously create for us. Meryl Streep and Robert De Niro will always be great performers but they will never be great stars, precisely because they disguise themselves so well.

For Roberts, the Audrey Hepburn analogy is a useful one. Like "Pretty Woman," "Roman Holiday" was a romantic comedy that took the form of an updated fairy tale (with Hepburn as a princess in disguise), and both films cast their fresh young female leads against established, stolid male stars not known for their sense of humor (Richard Gere versus Gregory Peck). And not only was Hepburn nominated for an Oscar for her work in what was, for all practical purposes, her screen debut, but she won it, too.

Self-consciously or not, both films frame their young stars with a mythology of discovery and invention - "Roman Holiday" with a variation on the "Cinderella" tale (though this time it is the princess who descends among the commoners), "Pretty Woman" with a combination of "Cinderella" and "Pygmalion" (a role Hepburn would return to with "My Fair Lady"). These are heroines who come along, seemingly out of nowhere, to redeem jaded, impassive and considerably older men. They are living signs of rejuvenation and revitalization - of new beginnings.

Roberts, of course, possesses none of Hepburn's European polish and sophistication. She remains a rough-hewn, native American, and audiences love her in direct proportion to her uncertainty, awkwardness and innocence. The opposite of the urbane, civilized Hepburn, Roberts attracts metaphors of nature and wildlife - she is not only coltish but skittish, frisky, gawky, frolicsome. Perhaps her real antecedent isn't Hepburn, but "Bambi."

In matters of dress, Roberts favors floppy hats, print dresses and oversized men's shirts. "Sleeping With the Enemy" even makes a plot point out of her taste in shapeless clothing: Her evil, controlling husband (Patrick Bergin) forces her to wear a tight, sexy cocktail dress to a party, while her new gentle lover (Kevin Anderson) treats her to a romp through the Victorian costumes in the local theater's collection. Roberts still seems uneasy and uncomfortable with her body, as though she were astonished to find herself so suddenly an adult.

But for all of Roberts' sweetness and light, her films consistently place her in contexts that project anything but. So far she has been a Hollywood hooker, a victim of fatal illness, a medical student experimenting with suicide, a girl from the wrong side of the tracks looking for a rich boy and now a battered wife. Her next project will be a Joel Schumacher film called "Dying Young." One thinks of the career of Ingrid Bergman, who arrived from Sweden hailed as a glowingly healthy, farm-fresh young star and was promptly cast as an adulteress ("Intermezzo," "Casablanca"), a nymphomaniac ("Notorious") and a prostitute ("Dr. Jekyll and Mr. Hyde," "Arch of Triumph").

But the specter haunting Roberts is not sex, but death. At a time when there are no studios to guide the careers and shape the images of their contract players, it's amazing that Roberts' dramatic identity has acquired such consistency and resonance from film to film. She has become that central allegorical image of high romantic painting and poetry.

It's intriguing that innocence should become such a priority with the American public at this point in time.