THE heroine of William Styron's ''Sophie's Choice'' is a creature of such extravagant and contradictory attributes that it isn't always easy, while reading the novel, to imagine her in the flesh. She is tragic, voluptuous, suddenly exuberant and then just as suddenly sodden, the survivor of one calamity and the woman at the heart of another. Mr. Styron's Sophie would seem too oversized and literary a figure to be embodied by any actress, even by an actress of extraordinary resourcefulness and versatility.

Meryl Streep has already established herself as a performer of that caliber, but nothing in her earlier work fully anticipates ''Sophie's Choice.'' In Alan J. Pakula's faithful screen adaptation of Mr. Styron's novel, Miss Streep accomplishes the near-impossible, presenting Sophie in believably human terms without losing the scale of Mr. Styron's invention. In a role affording every opportunity for overstatement, she offers a performance of such measured intensity that the results are by turns exhilarating and heartbreaking. Though it's far from a flawless movie, ''Sophie's Choice'' is a unified and deeply affecting one. Thanks in large part to Miss Streep's bravura performance, it's a film that casts a powerful, uninterrupted spell.

Mr. Pakula's ''Sophie's Choice,'' which opens today at Cinema I, follows the lengthy novel closely enough to capture it with amazing comprehensiveness in a little more than two-and-a-half hours. In fact, the novel is reflected so accurately that both its strengths and its weaknesses remain intact. Struggling as it does to compress the book, the film loses much of the novel's windiness, although a few voice-over narrative passages (read by Josef Sommer, as an older and wiser version of Mr. Styron's part-autobiographical character, Stingo) quickly recapitulate that aspect.

The manipulative and oversymmetrical aspects of the story are also here, and the events and devices that seemed awkward on the page - Mr. Styron's having Sophie and her lover Nathan dress in period garb to suggest their manic ebullience, for instance -have stayed that way. But the book's most overpowering quality is its inexorable momentum, and that has been preserved to the fullest. A suspenseful, troubling novel, it makes for a movie that is even more so.

The bulk of Mr. Styron's story is set at a Brooklyn boardinghouse in 1947, where the aspiring writer Stingo is befriended by two lovers, Sophie and Nathan, whose instability and flamboyance utterly capture his imagination. Sophie gradually tells Stingo, in a series of long confessional monologues, that she is a former Roman Catholic, an Auschwitz survivor and, finally, the woman with the terrible secret reflected in the title. She is also a liar, at least at first, and much of the book's suspense revolves around Stingo's search for her authentic story. As for Nathan, he is endowed by Mr. Styron with a terrible secret of his own and with a mixture of demonic rage and irresistible charm. Together, Nathan and Sophie storm at each other until Stingo is hopelessly caught up in their affair.

Among the more daring things Mr. Pakula attempts here is the structural anomaly of interrupting the Brooklyn drama with a lengthy Auschwitz flashback, midway through the story. He manages to make it work, both because Miss Streep so successfully holds the audience rapt and because the Auschwitz segment is so crucial and compelling. At the concentration camp, Sophie wheedles her way into a secretarial job at Rudolf Hoess's home, a bizarrely bourgeois oasis in which a child's room is decorated with swastika-pattern lace curtains and the mother of the house sings out ''I baked Himmler's favorite cake!'' That the film avoids a tone of crushing irony here and elsewhere is attributable to the strength, patience and delicacy with which Mr. Pakula has approached his material.

The film's two leading men, Kevin Kline as Nathan and Peter MacNicol as Stingo, have roles that are in some ways even more challenging than Miss Streep's. Mr. Kline, whose Nathan convincingly demonstrates the greatest of tenderness toward Sophie, is also called upon to rail at her mercilessly. In the tender scenes Mr. Kline makes himself very appealing; in the cruel ones, he does the best he can to affect a viciousness that, even on the page, seemed less than fully convincing.

Mr. MacNicol plays Stingo with a touching Southern gentlemanliness and reserve, but the role of an admiring listener has its difficulties, too. And he isn't helped by the novel's hubris regarding Stingo, preserved here in a passage in which Nathan only half-facetiously welcomes the aspiring young author into ''that pantheon of the gods whose words are all we know of immortality.'' The movie's Stingo, who hasn't demonstrated anything to warrant this, is called upon to take the compliment more or less in stride.

The chief thing Stingo must do, though, is to lead the reader or audience toward an unqualified fascination with Sophie. That part is easy. The character's halting, Polish-accented speech; her charming (and, in one instance, hilariously obscene) malapropisms; her frank sexuality (something Miss Streep conveys easily without any need for nudity); her long, haunted reminiscences -these are the components of an unforgettable heroine, and the work of the astonishing actress who brings her to life.

After Auschwitz

SOPHIE'S CHOICE, directed by Alan J. Pakula; screenplay by Mr. Pakula; director of photography, Nestor Almendros; editor, Evan Lottman; music by Marvin Hamlisch; produced by Mr. Pakula and Keith Barish; released by Universal Pictures. At Cinema 1, Third Avenue and 60th Street. Running time: 157 minutes. This film is rated R.

Sophie . . . . . Meryl Streep
Nathan . . . . . Kevin Kline
Stingo . . . . . Peter MacNicol
Yetta . . . . . Rita Karin
Larry . . . . . Stephen D. Newman
Leslie Lapidus . . . . . Greta Turken
Narrator . . . . . Josef Sommer
Morris Fink . . . . . Josh Mostel
Astrid Weinstein . . . . . Marcell Rosenblatt
Moishe Rosenblum . . . . . Moishe Rosenfeld
Lillian Grossman . . . . . Robin Bartlett
Polish Professor . . . . . Eugene Lipinski
Librarian . . . . . John Rothman
Rudolf Hoess . . . . . Gunther Maria Halmer
SS Doctor . . . . . Karlheinz Hackl
Frau Hoess . . . . . Ulli Fess
Emmi Hoess . . . . . Melanie Pianka
Wanda . . . . . Katharina Thalbach
Josef . . . . . Neddim Prohic
Sophie's children . . . . . Jennifer Lawn, Adrian Kalitka
Sophie as a child . . . . . Michaela Karacic