At the beginning of the pilot, Sondra, the Huxtables' eldest daughter, was added as an afterthought. However, when Clair Huxtable mentions the couple's four children, they decide to add another child to represent the results of a good upbringing. The casting came down to 26-year-old Sabrina Le Beauf and 21-year-old Whitney Houston. LeBeauf was too old for the role, only ten years younger than Phylicia Rashad. However, Houston refused to sign a contract committing her to the show as she wanted to pursue her singing career. This forced the producer's hand, and LeBeauf was given the role of Sondra.
Peter Costa, who played Huxtable neighbor and Rudy's playmate Peter, suffered from stage fright and often forgot his lines. As a result, his character's communication via trademark facial expressions and always running out of the house at the first sign of trouble.
Cliff was originally going to be a chauffeur. Bill Cosby's wife, Camille, suggested making him a doctor, more fitting with the show's agenda to shatter racial stereotyping.
Inspired by I Love Lucy (1951), Cosby intended Clair to be a Dominican who would go into Spanish tirades when she got angry. Phylicia Rashad, who spoke fluent Spanish from living in Mexico as a child, won the part by demonstrating her language skills. Although the idea never happened, Clair speaks Spanish in a few episodes.