When director Tony Bill was casting the part of Mary Tyler Moore's ballet-loving, leukemia-stricken daughter in the upcoming film Six Weeks, he launched an Annie-sized search for a teenage actress who could dance. After screening several thousand girls without success last fall, Bill tried the opposite approach and asked the New York City Ballet's Jacques D'Amboise to help him find a dancer who could act. D'Amboise did not hesitate to recommend Katherine Healy, a 13-year-old, five-foot, 85-pound sprite whom he calls "one in a million." Ironically, since Katherine has devoted most of her time to ballet, she has had little time for television or movies and hence she had never seen either MTM or co-star Dudley Moore. But after one-on-one interviews with them, Healy did a screen test and won the plum role in late October. "We had a lot of fun," Katherine now says of her three months on the set. "Mary and Dudley were always making jokes." The only child of a utilities corporation executive, New York-born Katherine started ice skating at 3. was spotted by Olympic gold medalist John Curry the following year, and by 6 was skating with Curry and Dorothy Hamill at an all-star benefit at Madison Square Garden. At 10, she was the subject of photographer Jill Krementz's top-selling A Very Young Skater. But she also had been smitten with ballet after seeing Rudolf Nureyev in the documentary I Am a Dancer In 1976 she began classes at New York's prestigious School of American Ballet. Since Six Weeks' filming ended last February, the eighth grader at St. Ann's School in Brooklyn Heights has resumed training with Gelsey Kirkland's former coach, David Howard. For now, she is turning down all TV and movie offers. "I'd rather be a dancer," she explains. Says Dudley "Ballet's gain is acting's loss."