Performances of Mikhail Baryshnikov's "Cinderella" by American Ballet Theatre last night and Thursday at the Kennedy Center Opera House were an object lesson in the perils and pleasures of live theater. While the secondary casts both evenings were essentially the same, the performances could not have been more different.

Thursday's was tired and leaden -- even the antics of the Step-Sisters seemed more annoying than wicked. The next night the ballet returned to life in a fresh and enjoyable rendition. The acting was clearer, the dancing was on the whole better, and the comedy was more outlandish. The Step-Sisters, danced both nights by David Cuevas and Thomas Titone, had returned to form, abetted by Raymond Serrano's repulsive Step-Mother.

The ballet also continues to undergo choreographic changes from night to night. Several bits of Act I stage business -- for example, Cinderella's sewing clothes for her Step-Sisters, and the stepfamily's shrinking from the disguised Fairy Godmother -- were clearly different over these performances.

Other more obvious fiddling is also being done by Baryshnikov and co-choreographer Peter Anastos. The most obvious of the changes is that the Step-Sisters -- roles played en travesti -- were not dancing on point Thursday or Friday (though they were at Wednesday's opening). Word from the company has it that this experimentation is being done to free up casting, since not all the men who could conceivably dance the role are capable of doing so on point.

The dramatic effect of this change is to emphasize the men-in-drag aspects of these roles, thereby thoroughly setting the Step-Sisters apart from Cinderella and the other women in the ballet. What is definitely lost by this action is the humorous impact of those ungainly creatures, who tower over the other women, attempting to be aerial. Clearly the roles were choreographed for point work and its loss is felt in the clarity of those gigantic "stilettos."

On Thursday the principal roles were taken by Cynthia Harvey and, making his Washington debut as the Prince, Robert La Fosse. This pairing was of a decidedly odd-couple nature. Harvey's independent and feisty Cinderella -- with wells of hidden strength feeding her luminous dancing -- would surely not be attracted to La Fosse's immature playboy Prince. La Fosse simply has not the ballon to carry off the Baryshnikovian pyrotechnics of the role, but more damaging to the role, his Prince was an emotional washout.

Friday's performance also paired opposites, though this seemed more a matter of a harmonious balance of temperaments. Where Marianna Tcherkassky's Cinderella was all yielding sweetness and purity, Danilo Radojevic was a commanding and energetic firebrand of a Prince. Tcherkassky made Cinderella seem appropriately downtrodden at home, escaping the situation into flights of fantasy, and her jubilation at her turn of fortune seemed real. Only a deficiency of quickness in her Act I variations marred her performance.

Cinderella continues through Sunday evening.