(written from a Production point of view)
Marjorie Ellen Pivonka Mahoney (born 20 February 1936; age 83), best known as Marj Dusay, is the Emmy nominated actress who played Kara in the Star Trek: The Original Series third season episode "Spock's Brain". She filmed her scenes on Monday 8 July 1968 and between Wednesday 10 July 1968 and Friday 12 July 1968 at Desilu Stage 9 and Paramount Stage 3.
Dusay made her film debut in 1967 playing a small role alongside Elvis Presley in the film Clambake in which Angelique Pettyjohn also appeared. During the 1960s and 1970s she made numerous appearances on popular televisions shows such as Wild Wild West, Bonaza, Hawaii Five-0, Hogan's Heros, The Bionic Woman, and The Streets of San Francisco. Other than Star Trek, her only notable role in Science Fiction was a brief appearance in the Battlestar Galactica spin-off series Galactica 1980.
Since the 1980s, Dusay has been best known for her performances on American daytime soap operas. From 1983 until its cancellation in 1987 Dusay played Myrna Clegg on Capitol, a soap opera which took place in Washington DC. From 1987 until 1991 she played Pamela Capwell on Santa Barbara. From 1992 to 1993 she took the role of Vivian Alamain on Days of Our Lives, replacing an unavailable Louise Sorel.
Starting in September 1993, she took on her best-known role as Alexandra Spaulding on Guiding Light, taking over the role from Beverly McKinsey. She left the role in March 1997, but returned for a second time in 1998 and 1999. After being briefly replaced by Joan Collins in 2002, she took the role for a third time and remained with it until the series' cancellation in 2009. 
Between 1998 and 2002, she portrayed the role of Vanessa Bennett Cortland in another soap opera, All My Children, playing the character at the same time as Lee Meriwether, Olivia Birkelund and Samantha Eggar worked on the series.
She also appeared in a 1969 episode of Daniel Boone and a 1970 episode of Family Affair (with Star Trek: Deep Space Nine guest Brian Keith). For many years she worked with Michael Zaslow who played the villain Roger Thorpe on Guiding Light.
According to an audio interview on her official website she did not watch her Star Trek episode until a fan sent her a VHS tape in the 1990s. The many autograph requests she had received over the years had led her to believe that the episode was a popular one. She describes in mock horror watching her performance and said "Brain and Brain, what is Brain...how the hell did I get that job". Nevertheless she describes having fun making "Spock's Brain" and specifically that she enjoyed her costume and the impressive boots created for her.
Although she occasionally engages in public appearances related to Star Trek or her soap opera career, Dusay effectively retired from acting following the cancellation of Guiding Light. In 2017 she disclosed that her retirement was reinforced by complications from back surgery which left her with limited mobility.