For most of the scenes featuring both Patty and Cathy, actress Rita Walter played "the back of either Patty's or Cathy's head", as appropriate. She can also be seen in several episodes as a background character.
Patty Duke was only 16 when production began and thus it was decided to film the show in New York at the Chelsea Studios in Manhattan. New York's child labor laws were more liberal than California's--especially that state's Coogan Law (named after child actor Jackie Coogan), which regulated the working conditions of child actors. Filming in New York allowed Duke to work more hours per day, an advantage since she pretty much carried the show. Duke turned 18 during filming of the final (1965-66) season, and although the season started in New York, the whole production was moved to Hollywood by season's end much to the chagrin of Duke, who wished to stay in New York. Nevertheless, the setting of the series remained the same Brooklyn Heights neighborhood it had always been.
Although the series was still doing well enough in the Nielsen ratings, ABC decided not to renew it for a fourth season. ABC wanted all of their black & white shows to switch color production. United Artists wanted more money to make the change. The network decided it would be more cost-effective to develop a new color show instead.
Besides the differences mentioned in the theme song, Patty's eyes were always opened wide and she spoke much louder and faster than Cathy who had a slight English accent and spoke more quietly than Patty and Cathy's eyes were not opened as wide. Patty had the bangs while Cathy combed her hair back.
Patty Duke (Patty Lane / Cathy Lane), William Schallert (Martin Lane) and Eddie Applegate (Richard Harrison) all died in 2016: Duke on March 29, Schallert on May 8 and Applegate on October 17. With Applegate's death, Paul O'Keefe (Ross Lane) is the last surviving regular cast member of the series.
In an EmmyTvLegends interview Patty Duke says that the Cathy and Patty Lane characters were developed by writer/producers William Asher and Sydney Sheldon who were basically spinning off aspects of her own personality. She admits the polar opposite twins were basically aspects of and expressions of her own bipolar personality.