Dame Margot Fonteyn: still charming her audiences

She is a trim figure, sitting - ankles crossed, back straight - in the dancewear department of Filene's, a Boston department store, autographing posters. Above her table, two video screens show pulsing bodies in Danskin leotards doing ''dancercize,'' but she sits almost primly, her head bobbing up after each signature, alert and bright-eyed as a robin, to give everyone who takes a poster a brilliant smile.

She's wearing a flat, red hat tipped jauntily to one side and a trimly belted black-checked dress with a bunch of shiny red cherries at one shoulder. That smile is so firmly bright, she reminds one of Mary Poppins. It's a magical smile , and it's fitting that the poster Margot Fonteyn is signing is for a public broadcasting series she narrated called ''The Magic of Dance.''

It's the way she always smiled when she finished a pirouette. Her whole body seemed to smile. Critics saw it and called her ''ageless.'' Audiences leaped to their feet and roared. She remembers that her most famous partner, Rudolf Nureyev, ''would never quite be able to understand why I could do my little dance in my rather pitiful little way and get a great deal of applause and he, on the other hand, had to do 25 huge leaps to get applause.''

She thinks his leaps got audiences in a good mood, so they applauded her, too. But that's not it. It's a sense of goodness and, somehow, rightnessm in the very English way she danced. Whirling out of a faultless pirouette and coming around to face the audience beautifully upright, or collecting herself from the stretch of a lyrical arabesque, there was that smile. It's a good-little-girl smile, very gracious, but also expressing a sense of satisfaction in having done well.

I mentioned her smile, and she explained, in the crisp, almost ballet-mistressly tones in which she narrates ''The Magic of Dance,'' that it dates from the 19th century.

''I suppose I'm more of a 19th-century dancer than a 20th-century dancer - if you have to choose between the two - because my teachers when I was young were, many of them, the old ballerinas from the end of the 19th century. They were born in 1890. . . . So the atmosphere of my training was of a period when you go out on the stage, and you smile at the audience, and you kind of dance to the audience. In classes, they would say, 'Look at the public.' My first dancing teacher would say, 'Smile, smile'm '' (she is whispering, and you can just see the four-year-old Peggy Hookham obeying diligently) ''because this was the way of dancing at the time. You presented yourself to the audience in the most charming way you could.''

Those teachers were mostly Russian emigres. Her family had to go to Shanghai for her father's business, so she was able to study with George Gontcharov of the Bolshoi. When she returned to England, she entered the school of the Old Vic and Sadler's Wells theaters, and in 1935, at age 16, was dancing ''Giselle'' in the Sadler's Wells company (which later became the Royal Ballet).

In an interview, she continued, as charmingly as possible, to talk about the virtues of 20th-century dancing and how Martha Graham brought modern dance into focus by dancing from within. ''One phrase she had which really impresses me very much was 'the inner landscape.' ''

Her dancing was selfless. Instead of presenting her own inner landscape, she ''put herself into the skin'' of the character she was playing. ''If I was doing 'Giselle,' I was Giselle.'' She danced ''Romeo and Juliet,'' ''starting out as Juliet and not knowing what the rest of the story would be. Just go out onstage and then gradually go through it . . . as though it were happening for the first time.'' She added that she always wept when her character died, and loved to lie ''abandoned, with my hair down, on the stage, weeping,'' enjoying shamelessly a particularly sad ending as much as the audience.

When she danced ''Giselle,'' she began a professional career that spanned the length of two normal ballerinas' careers. She defined the style of the English ballerina, and made it famous, when the company made its triumphal 1949 appearance in New York. In 1956, she was made a dame by Queen Elizabeth. Later that same year, the Sadler's Wells and the Bolshoi exchanged performances, lifting the iron curtain on ballet. She was already prima ballerina assolutam when a young Russian upstart, Rudolf Nureyev, defected and requested her as a partner. The magic began all over again. ''I think that was one of those funny things that worked out well,'' she says. ''If we had been the same age, maybe it wouldn't have worked at all. It was just, perhaps, that he had a certain respect for me because I was much older and was already famous, and I had a respect for him because he was this extraordinary . . . I thought, 'What am I doing with this thing?' ''

She doesn't dance anymore, but lives on a farm in Panama with her husband, Roberto Arias, a former diplomat.

She declares bluffly, in her low British voice with its deep, hidden giggle, that she is old and shouldn't have danced so long, but she felt it was her destiny. Later, she says she thinks high leaps are the prerogative of male dancers and she doesn't approve of female acrobatics in ballet - ''and not just because I couldn't get two inches off the floor.'' She is, as usual, giving, and it is a delight to receive her statements about herself and her art. They are as straightforward and to the point as her exquisite pirouettes were. They also have a spin on them: Even as you admire her for her honesty, you don't believe her for a minute, because she is charming you with the skill of a prima ballerina assolutam and the ageless wisdom of a 10-year-old.