In the most famous scene in Jill Clayburgh's most influential movie, her character reacted to the news that her husband wanted to leave her. Clayburgh's Erica responded with such naturalness, confusion and wounded pride that she captured the imagination of a generation.

"As Miss Clayburgh plays this scene," Vincent Canby wrote about "An Unmarried Woman" in 1978, "one has a vision of all the immutable things that can be destroyed in less than a minute, from landscapes and ships and reputations to perfect marriages." But Clayburgh proved that a reputation could be made in less than a minute, too.

Has any actor's career ever been more powerfully affected by a prefix? It was the "un" in "Unmarried" that established Clayburgh's creative power. Women's roles had been changing irrevocably, and a new assertiveness was being established and understood. But the usual story lines of that era followed female characters' quests for independence and authority. Heroines rebelled. They picked themselves up and moved out. They took action. They weren't acted upon.

Their roles were sharply defined, but Erica's was not. Writer and director Paul Mazursky had a divorced friend who described herself as "an unmarried woman" on a mortgage application. Extrapolating from that, he envisioned the story of a Manhattan wife set adrift. But Clayburgh's shaping of the character was unmistakably her own, just as surely as its impact on female movie audiences was universal. The unaffected nature of her performance became its most distinctive feature. She didn't have the tics of Diane Keaton, the steel of Jane Fonda, the feistiness of Sally Field, the uncanny adaptability of Meryl Streep. She simply had the gift of resembling a real person undergoing life-altering change.

"Mr. Mazursky has written a marvelous role for the actress, so I suppose it's not unfair of him to depend on her to carry the movie," Canby wrote. Carry it she did.

Clayburgh, who died at her Connecticut home Friday at 66 after living with chronic leukemia for 21 years, had been on stage and screen for a decade before giving this definitive performance. But she could be awkwardly miscast and at first often was. She was blond and beautiful, but she was about as much like Carole Lombard as James Brolin was like Clark Gable ("Gable and Lombard," 1976). Without "An Unmarried Woman" she might never have found her niche.

But once she did, she began a streak. She went from playing an opera star in Bernardo Bertolucci's 1979 "Luna," one of the most conversation-stopping films ever to open the New York Film Festival. She made widely seen comedies about smart, interesting women ("Starting Over" in 1979, "It's My Turn" in 1980). She even turned up on the Supreme Court ("First Monday in October" in 1981), a likable presence even in highly unlikely circumstances. "The FBI is wrong in reporting to you that I have no children," she told the cinematic senators. "Ideas are my children, and I have hundreds of them."

Then she and her husband, the playwright David Rabe, had real children, Lily and Michael. Although Clayburgh kept working, her public presence grew more intermittent, the available film roles more motherly or eccentric. (She appeared in the 2006 film version of Augusten Burroughs' "Running With Scissors.") She was so greatly missed that any major appearances were apt to be described as comebacks (two TV series in the late '90s, "Barefoot in the Park" on Broadway in 2006), but the roles that should have been welcoming hardly existed anymore. Only in life did we wonder what had become of those Ericas 30 years later.

She remained elegant, lovely and so recognizable that she became accustomed to being treated as an avatar. "My God, you've defined my entire life for me," one "Unmarried Woman" fan told her in 2002, and that experience was apparently not unusual for her. When she and Lily, an actress, roomed together in Manhattan in 2005 as both prepared for stage appearances, a writer for The New York Times visited the eternal heroine and still saw her unforgettable movie persona.

"Jill Clayburgh appears to be living in an updated Jill Clayburgh vehicle," Nancy Hass wrote. "Fluttery-yet-determined mom flees comfortable exurban married life to share tiny Manhattan apartment of headstrong, aspiring-actress daughter. Conflict, hilarity and, of course, self-actualization ensue." For Clayburgh, in life and work, that's just what happened.