(written from a Production point of view)
This is a list of performers who were considered for roles in the Star Trek franchise, but ultimately did not appear in the role in the final episode or film. Performers listed here have been verified as having been considered by Star Trek personnel for a particular role on Trek in which they ultimately did not appear.
By series Edit
- See: Performers considered for TOS roles
- See: Performers considered for film roles
- See: Performers considered for TNG roles
- See: Performers considered for DS9 roles
- See: Performers considered for VOY roles
- See: Performers considered for ENT roles
- See: Performers considered for DIS roles
By actor Edit
- Main article: David Gautreaux
David Gautreaux (born 28 June 1951; age 65) was set to play the role of Vulcan science officer Lieutenant Xon in the second Star Trek series, but the planned show was cancelled. Finally he played Commander Branch in Star Trek: The Motion Picture. (Phase II: The Lost Enterprise on Star Trek: The Motion Picture (The Director's Edition) DVD)
Yaphet Kotto Edit
Kotto is known for his numerous film roles, including Mr. Big in Live and Let Die, Parker in Alien (1979), William Laughlin in The Running Man (1987, with Mick Fleetwood), and FBI Agent Alonzo Mosely in Midnight Run (1988). He is also known for his role as Lt. Al Giardello on the NBC drama series Homicide: Life on the Street.
In addition, Kotto co-starred with TOS actress Nichelle Nichols in the film Truck Turner. Kotto's other film credits include 1968's The Thomas Crown Affair, 1970's The Liberation of L.B. Jones (with Anthony Zerbe), 1978's Blue Collar (with Ed Begley, Jr.), 1979's Alien, 1983's The Star Chamber (with Larry Hankin and James B. Sikking), and 1994's The Puppet Masters (with Julie Warner, Sam Anderson, J. Patrick McCormack, Andrew Robinson, and Michael Shamus Wiles). He has also guest-starred on such TV shows as Bonanza, Gunsmoke, Hawaii Five-O (with Jeff Corey), Fantasy Island (starring Ricardo Montalban), and The A-Team (starring Dwight Schultz). In 1977, he received an Emmy Award nomination for his performance in the TV special Raid on Entebbe, which co-starred Tige Andrews, Robin Gammell, Stephen Macht, and David Opatoshu.
Roddy McDowall Edit
Roddy McDowall (1928 – 1998) was casting director Joseph D'Agosta's choice for the role of Trelane in "The Squire of Gothos". He was overruled by producer Gene L. Coon's choice, William Campbell who seemed to be more suitable for the part.  McDowall was also a favorite of director Joseph L. Scanlan for voicing Armus in the Star Trek: The Next Generation first season episode "Skin of Evil". (Creating the Next Generation: The Conception and Creation of a Phenomenon, p.60)
McDowall was an acclaimed character actor, who began his career in Hollywood as a child, acting in countless films from 1938 onward. His big break came at the age of 15 with the starring role in Lassie Come Home (1943). He also appeared in Orson Welles' 1948 film version of Macbeth, which also featured Morgan Farley. Afterwards, he appeared in numerous film and television roles, including The Longest Day (1962, with Jeffrey Hunter and John Crawford), Cleopatra (1963, with John Hoyt), Pretty Maids All in the Row (1971, with James Doohan, William Campbell, Dawn Roddenberry and written by Gene Roddenberry), The Poseidon Adventure (1972, with John Crawford, Bill Catching, and George Sawaya), and Funny Lady (1975). McDowall also voiced the role of V.I.N.CENT, a robot in Disney's The Black Hole (1979). McDowall is probably best known for his roles in four out of the five Planet of the Apes movies (which also featured James Daly, Lou Wagner, Paul Lambert, Billy Curtis, Jane Ross, Ricardo Montalban, William Windom, Jason Evers, Walker Edmiston, Janos Prohaska, James B. Sikking, Paul Comi, France Nuyen, Paul Williams, and David Gerrold), and the subsequent television series (which co-starred Mark Lenard).
On television, McDowall appeared in a famous 1960 episode of The Twilight Zone entitled "People Are Alike All Over", which co-starred Paul Comi, Susan Oliver, Byron Morrow, and Vic Perrin. He also appeared as "The Bookworm" in two 1966 episodes of Batman, which also featured John Crawford, and guest-starred in a 1972 episode of Columbo with James Gregory, William Windom, and George Sawaya.
Toshiro Mifune (1920 – 1997) was a legendary Japanese actor appearing in almost 170 movies, best known for his roles in director Akira Kurosawa's Samurai epics. Mifune was planned to play the main Klingon villain in the proposed Star Trek movie, Planet of the Titans in 1976. (The Star Trek Compendium)
Director/screenwriter Philip Kaufman said, "My version was really built around Leonard Nimoy as Spock and Toshiro Mifune as his Klingon nemesis... My idea was to make it less "cult-ish", and more of an adult movie, dealing with sexuality and wonders rather than oddness; a big science fiction movie, filled with all kinds of questions, particularly about the nature of Spock's [duality]-exploring his humanity and what humanness was. To have Spock and Mifune's character tripping out in outer space. I'm sure the fans would have been upset, but I felt it could really open up a new type of science fiction." 
Edward James OlmosEdit
Edward James Olmos (born 27 February 1947; age 70) is an Emmy Award-winning, Academy Award-nominated actor. Leonard Nimoy expressly wanted Olmos to play Kruge in Star Trek III: The Search for Spock, but the role ultimately went to Christopher Lloyd. (Star Trek III: The Search for Spock (Special Edition) special features)
A few years later, Olmos was offered the role of Jean-Luc Picard on Star Trek: The Next Generation. (Battlestar Galactica Panel, FedCon XIV, 24 May 2015; ) He had to turn it down as he was busy with other projects. His Hollywood Walk of Fame star is right next to Patrick Stewart's.
On film, Olmos is known for playing Gaff in the 1982 sci-fi thriller Blade Runner and for his Oscar-nominated role in 1988's Stand and Deliver. He received two Emmy nominations – winning his first – for his supporting role as Lt. Martin Castillo on Miami Vice. Now he has acquired new fame for his portrayal of Admiral Adama in Ronald D. Moore's hit re-imagining of the sci-fi series Battlestar Galactica, which originally aired on SyFy.
As it turned out, Olmos has been on record of being less than enamored with the science fiction genre, particularly for its portrayal of aliens, and having been seriously considered for the part of the Klingon Kruge was therefore somewhat ironic. Olmos only accepted his role in Battlestar Galactica when it became apparent to him that the series was to be primarily drama-driven and not to feature any "outlandish" life forms. (Battlestar Galactica: The Complete Series Blu-ray-special features, "Cast And Crew Take A Look Back"; "The Last Frakkin Special")