(written from a Production point of view)
Janet MacLachlan (27 August 1933 – 11 October 2010; age 77) was the actress who played Lieutenant Charlene Masters in the Star Trek: The Original Series first season episode "The Alternative Factor". She filmed her scenes for the episode on Wednesday 16 November 1966, Friday 18 November 1966 and Monday 21 November 1966 at Desilu Stage 9.
MacLachlan was born in New York City and graduated with a bachelor's degree in psychology from Hunter College in 1955. She studied acting at the Harlem YMCA, the Herbert Berghoff Acting Studio, and the Little Theatre of Harlem. 
She began her acting career with small stage roles which led to her joining the Guthrie Theater in Minneapolis, Minnesota. Soon thereafter, she moved to Los Angeles and further trained at The Actors Studio, Joanie Gerber Voiceovers, and Theatre East. In 1964, she became a contract player for Universal Studios.
She made her television acting debut on The Alfred Hitchock Hour in 1965. She appeared in two episodes that year, the second of which featured Jane Wyatt. In 1966, MacLachlan appeared in two episodes of The F.B.I., on which Stephen Brooks was a regular. Her second episode was directed by Joseph Sargent and also featured Fritz Weaver. MacLachlan made a third appearance on The F.B.I. in 1968; Stephen Brooks had left the series by this time, but the episode guest-starred Monte Markham and Michael Strong.
Perhaps MacLachlan's most notable television role in the 1960s was that of Laya in the 1967 I Spy episode of the same name. In the episode, MacLachlan played the love interest of Bill Cosby's character, Alexander Scott. Keith Andes also appeared in the episode. Later, in 1969, MacLachlan appeared on The Mod Squad as the secretive girlfriend of Linc Hayes, played by Clarence Williams III. Both Williams and Tige Andrews were regulars on this series.
Other shows on which MacLachlan appeared throughout the late 1960s include The Fugitive (with Ted Knight and William Sargent), Run for Your Life (directed by Leo Penn and co-starring Tige Andrews and James B. Sikking), The Girl from U.N.C.L.E. (with Anthony Caruso), and Ironside (with James Gregory, Ken Lynch, and Jason Wingreen).
MacLachlan was most prominent on television in the 1970s. In 1970, she was reunited with Bill Cosby on The Bill Cosby Show, on which she also worked with Michael Ansara. She followed this with appearances on The Name of the Game (with George Murdock) and Longstreet (with Robert DoQui, Brock Peters, and Logan Ramsey). MacLachlan worked with both Murdock and DoQui again in a 1974 episode of The Streets of San Francisco, in which she and Robert Hooks played husband and wife. (Harvey Jason and Ron Glass were in the episode, as well.)
MacLachlan's TV credits during the 1970s also included appearances on The Rockford Files, Mary Tyler Moore (with the aforementioned Ted Knight), Griff (starring Vic Tayback), Medical Center (with Barbara Baldavin, James Daly, and Thalmus Rasulala), S.W.A.T. (with Dallas Mitchell, Warren Munson, and Eric Server), The Six Million Dollar Man (directed by Cliff Bole), The Blue Knight (directed by Ralph Senensky), Barney Miller (with Ron Glass, Jon Lormer, and Kenneth Tigar), Wonder Woman (with Tim O'Connor and Vic Perrin), and All in the Family (with Jason Wingreen).
In addition to her guest appearances, MacLachlan acted in several made-for-TV movies in the 1970s. In 1972 she played a crucial role in Sounder as the schoolteacher who takes a wandering boy under her wing. In 1976's Louis Armstrong – Chicago Style, she portrayed the second wife of the title jazz legend, who was played by Ben Vereen. That same year, she was seen in Robert Butler-directed Dark Victory, along with the aforementioned Eric Server and Vic Tayback. She also starred in 1978's Roll of Thunder, Hear My Cry, which was written by Arthur Heinemann.
In 1980, MacLachlan made three appearances as housekeeper Polly Swanson on the sitcom Archie Bunker's Place, on which Bill Quinn and Jason Wingreen were regulars. The following year, MacLachlan participated in the PBS special Voices of Our People: In Celebration of Black Poetry, for which she won a Los Angeles-area Emmy Award. From 1982 through 1987, MacLachlan played the recurring role of Lynne Sutter on the police drama Cagney & Lacey (working with Stanley Kamel, Michael Pataki, Dan Shor, Gregory Sierra, Harvey Vernon, and director Alexander Singer). She also had a recurring role on the soap opera Santa Barbara in 1985, when Judith Anderson was also a regular cast member on that show. Others she worked with on this series include Nicolas Coster, Judith McConnell, and Louise Sorel.
In 1985, MacLachlan made the first of two guest appearance on the mystery drama Murder, She Wrote, working alongside Adrienne Barbeau and Susan Oliver. (Her second appearance was in 1994.) MacLachlan also made one-time appearances on such shows as Quincy, M.E. (with Robert Ito and Garry Walberg), Fantasy Island (starring Ricardo Montalban, in an episode with LeVar Burton), Hill Street Blues (with Barbara Bosson, John Chandler, Charles Cooper, Miguel Ferrer, Louis Giambalvo, Phillip Pine, James B. Sikking, and David Spielberg), Trapper John, M.D. (with Madge Sinclair and Ray Walston), L.A. Law (with Karen Austin, Corbin Bernsen, Barbara Bosson, and Bruce French), Our House (with Chad Allen and Christopher McDonald), Moonlighting (with Gwen Van Dam), Beauty and the Beast (with Theodore Bikel, Ellen Geer, and star Ron Perlman), and Murphy Brown (with Steven Culp).
MacLachlan appeared in several TV movies in the early 1980s, including The Sophisticated Gents (with Bibi Besch, Bernie Casey, Albert Hall, Robert Hooks, Thalmus Rasulala, Davis Roberts, Paul Winfield, and Alfre Woodard), Jacqueline Susann's Valley of the Dolls (starring Catherine Hicks and featuring Byron Morrow and Jean Simmons), The Kid from Nowhere (with Rene Auberjonois), and For Us the Living: The Medgar Evers Story (again working with Thalmus Rasulala and Paul Winfield). Later in the decade, her TV movie credits included Toughlove (with Helene Udy), Baby Girl Scout (with Julie Cobb, Ronny Cox, Raphael Sbarge, and Armin Shimerman), and Killer Instinct (with Roy Brocksmith and William Marshall).
1990s and 2000s
In 1994, MacLachlan made a return appearance on Murder, She Wrote, in an episode with Star Trek: Voyager regular Robert Beltran and Marta DuBois. She later had a recurring role as Mrs. Latrell on Murder One, during which time she worked with Geoffrey Blake, Barbara Bosson, John Fleck, April Grace, Gregory Itzin, Jack Kehler, Stephen Lee, Don McManus, Clayton Rohner, Don Stark, and Rick Worthy. Other TV shows on which she appeared in the 1990s include Father Dowling Mysteries (directed by Robert Scheerer and co-starring Roy Brocksmith and David Selburg), In the Heat of the Night (with George D. Wallace and Jason Wingreen), ER (with Jennifer Gatti and Lily Mariye), NYPD Blue (with Gordon Clapp, Sharon Lawrence, and Diane Salinger), and Home Improvement (with Jim Beaver).
Some of MacLachlan's 1990s TV movie credits include A Family for Joe (with Barbara Babcock and Nikki Cox), Something to Live for: The Alison Gertz Story (with Victor Brandt and George Coe), and The Tuskegee Airmen (with Tim Kelleher, Daniel Hugh Kelly, Ed Lauter, Christopher McDonald, and Ned Vaughn). She also appeared in the 1992 mini-series Tracks of Glory, along with Phil Morris.
Following the turn of the century, MacLachlan appeared as a judge in two episodes of the drama Family Law, which starred Christopher McDonald, Salli Elise Richardson, and Julie Warner. In 2002, MacLachlan made her final television appearance in an episode of Alias, the spy drama created by J.J. Abrams. The episode in question, "Cipher", was written by Alex Kurtzman and Roberto Orci (who were also co-executive producers on the series) and also featured Victor Garber and Terry O'Quinn.
MacLachlan made her feature film acting debut in the 1968 Paramount Pictures drama Up Tight!, which also featured Robert DoQui and Alan Oppenheimer. The following year, MacLachlan co-starred with fellow TOS guest actress Susan Oliver in the science fiction drama Change of Mind.
Three films released in 1970 featured MacLachlan: …tick… tick… tick… with Bernie Casey; Halls of Anger with Roy Jenson, Kim Manners, John McLiam, and Davis Roberts; and Darker Than Amber with Theodore Bikel. She also had a supporting role in the 1972 Joseph Sargent-directed drama The Man, which was made as a TV movie but ultimately released in theaters. This film also featured Robert DoQui, Vince Howard, Barry Russo, Garry Walberg, and MacLachlan's fellow TOS guest star, William Windom.
MacLachlan had a key supporting role as a teacher in the Academy Award-nominated 1972 film Sounder, which starred Paul Winfield. The following year, she again worked alongside Star Trek: Deep Space Nine guest actor Bernie Casey in the biographical drama Maurie. Her next feature was the 1984 Clint Eastwood thriller Tightrope, which also starred Geneviève Bujold. She later appeared in the action thriller Murphy's Law and the fantasy film The Boy Who Could Fly, both released in 1986 and each featuring another Trek guest star (Lawrence Tierney and Louise Fletcher, respectively).
In addition, MacLachlan and Ellen Geer both had a small roles in the 1987 comedy Big Shots, which co-starred Robert Joy and Paul Winfiield (his fourth project with MacLachlan after Sounder and two TV movies). MacLachlan was also seen in the romantic comedy For Keeps? the following year, as were Larry Drake and Kenneth Mars. In the 1990s, MacLachlan appeared in such films as Heart and Souls (with Kurtwood Smith, David Paymer and Alfre Woodard), There Goes My Baby (with Seymour Cassel and Andrew Robinson), Criminal Passion (with Henry Darrow), and Pinocchio's Revenge (with Ron Canada, Larry Cedar, and Aaron Lustig). Her last major feature film was 1999's The Thirteenth Floor, a science fiction thriller which also features Bob Clendenin, Brad William Henke, Darryl Henriques, Leon Rippy, and Jeremy Roberts.
MacLachlan retired from acting in 2002. She served many years as chair of the grant committee for the Academy of Motion Picture Arts and Sciences, a post she held as late as 2005. She died on 11 October 2010 at Kaiser Permanente Medical Center in Los Angeles, California after suffering a "cardiovascular incident". She was 77 years old.  MacLachlan was remembered at the 63rd Primetime Emmy Awards on 18 September 2011.