(written from a Production point of view)
William "Bill" J. Campbell (30 October 1923 – 28 April 2011; age 87),  from Newark, New Jersey, was the actor known to Star Trek fans for his portrayal of the god-child Trelane in the Star Trek: The Original Series first season episode "The Squire of Gothos" and for his role as Koloth in the second season episode "The Trouble with Tribbles". He reprised the Koloth role in the Star Trek: Deep Space Nine second season episode "Blood Oath", becoming one of the few actors to portray the same character on both the original series and on one of the spin-off series. (The character Koloth also appeared in a Star Trek: The Animated Series episode, but in that case, James Doohan provided the voice). He described his role on "Blood Oath" as his most difficult acting job, and one he would have liked to do again. (Star Trek: The Magazine Volume 1, Issue 13, p. 52) Campbell also reprised his role as Trelane in the video game Star Trek: Judgment Rites. His Klingon teeth from "Blood Oath" were once sold off by auction. 
Campbell filmed his scenes for "The Squire of Gothos" on Monday 31 October 1966 at Desilu Stage 9, and between Tuesday 1 November 1966 and Monday 7 November 1966 at Stage 10. He filmed his scenes for "The Trouble with Tribbles" on Thursday 24 August 1967 and Friday 25 August 1967 at Stage 10.
Later friend, Original Series Art Director Matt Jefferies, noted this on his performance as Trelane in 2002, "Of course, the saving grace was of course Bill Campbell that played the Squire. I think the way he took that part on, he probably wouldn't have missed it, if the set has disappeared." (TOS Season 2 DVD-special feature, "Designing the Final Frontier")
Campbell had always reveled in his Star Trek fame and had lent his presence to numerous events in the Star Trek convention circuit for decades. Even when his health started to fail him, starting in the early 2000s, he found the strength to attend his very last one, the Creation Entertainment 40th anniversary Star Trek convention held at the Las Vegas Hilton in August 2006, still managing to keep the audience "spellbound". 
Campbell was a close personal friend of Leonard McCoy actor DeForest Kelley whose death in 1999 came to him as a great personal shock. (Star Trek: The Magazine Volume 1, Issue 13, p. 53) Taking center stage, Campbell spoke affectionately of his deceased friend in the DeForest Kelley: A Tribute-special feature on the 2004 DVD release of Star Trek VI: The Undiscovered Country, reissued on the 2009 Blu-ray version. He was also a close friend of James Doohan, and attended his second wedding in 1967, and writer-producer Gene L. Coon, who came up with the idea of casting him as Trelane.
Career outside Star Trek
Campbell worked steadily in films and on television through the 1950s and 60s, beginning with the 1950 film noir The Breaking Point. In this film, Campbell appeared with fellow TOS guest performers Sherry Jackson and Peter Brocco. This was followed with supporting roles in an extensive number of movies, including Operation Pacific (1951), Battle Circus (1953), Escape from Fort Bravo (1953), The High and the Mighty (1954, co-starring Paul Fix and William Schallert), Battle Cry (1955, with Perry Lopez), and Man Without a Star (1955, with George D. Wallace).
Although Campbell received acclaim for his first starring role as a prisoner awaiting execution in the 1955 crime drama Cell 2455 Death Row, it did little to advance his career and returned to doing supporting roles. As one of the principal actors in 1956's Love Me Tender, Campbell became the first actor to sing with Elvis Presley in a film. Campbell then performed in such films Eighteen and Anxious (1957, with Yvonne Craig), and The Naked and the Dead (1958, also featuring Grace Lee Whitney). Campbell became a regular on the Canadian TV series Cannonball, which lasted thirty-nine episodes between October 1958 and July 1959.
By the 1960s, he began to appear more prominently on television, although he continued acting in low-grade films. In 1963, he was given the lead role in Francis Ford Coppola's 1963 cult horror movie Dementia 13, having worked with Coppola on Roger Corman's The Young Racers the year before. In 1964, Campbell appeared in Corman's war drama The Secret Invasion and had a supporting role in the Academy Award-nominated classic Hush... Hush, Sweet Charlotte. Campbell also had an uncredited role in 1965's The Money Trap, starring Ricardo Montalban. In 1966, Campbell starred as a vampire artist who kills women and places their bodies within his sculptures in Roger Corman's Blood Bath, co-starring Biff Elliot and Sid Haig.
Campbell's television credits during this time included appearances on Philip Marlowe (in an episode with Barry Atwater), Perry Mason (including an episode with Kenneth Tobey), The Wild Wild West (in an episode with Maggie Thrett, written by Gene L. Coon), Combat!, Bonanza, and Gunsmoke (including one episode with James Gregory and Ed McCready, directed by Vincent McEveety). In addition, TOS director Herschel Daugherty cast and worked with Campbell in a 1968 episode of It Takes a Thief! with Meg Wyllie and a 1969 episode of Bracken's World.
In 1971, Campbell co-starred with TOS regular James Doohan in Pretty Maids All in a Row, a comedy written and produced by Trek creator Gene Roddenberry. Dawn Roddenberry also had a small role in the film. The following year, Campbell co-starred with Bernie Casey in the blaxploitation classic Black Gunn, which was Campbell's last feature film credit, although he continued to act on television. He appeared with his TOS co-star George Takei and "Trouble with Tribbles" co-star Stanley Adams in a 1971 episode of the short-lived drama O'Hara, U.S. Treasury, written by Gilbert Ralston. This was followed with appearances on Ironside (with Antoinette Bower), Emergency! (with Vince Howard, Ken Lynch, and Kevin Tighe, directed by Joseph Pevney), Adam-12 (with Vic Perrin), The Manhunter (with William Smithers), another episode of Gunsmoke, Marcus Welby, M.D., Medical Center (with Barbara Baldavin and Louise Sorel, directed by Joseph Pevney), The Streets of San Francisco (with Darleen Carr, Jason Evers, and Ken Lynch), and The Hardy Boys/Nancy Drew Mysteries (with William Schallert, and directed by Michael Pataki).
In 1983, Campbell appeared in a two-part episode of Quincy, M.E., which featured Robert Ito and Garry Walberg as regulars, and was co-written and directed by Jeri Taylor. In 1985, he co-starred with Barbara Babcock and Richard Kiley in an episode of Hotel. In 1987, he co-starred with Gary Lockwood in the TV special The Return of the Six Million Dollar Man and the Bionic Woman. Campbell's work in the 1994 DS9 episode "Blood Oath" and a 1996 appearance in an episode of Kung Fu: The Legend Continues were his final two acting roles.
A lesser known aspect of Campbell were his charity efforts, especially the fund raisings for the "Motion Picture & Television Fund", a charitable organization that offered assistance and care to those in the motion picture industry with limited or no resources, when struck with infirmity and/or in retirement age. To this specific end he founded "FantastiCon" in 1996, an annual science fiction and fantasy convention, which celebrated the achievements of production staffers in these genres, complete with award presentations and ceremonies. All proceeds went to the Motion Picture & Television Fund. (Beyond the Clouds, p. 274: Star Trek: The Magazine Volume 1, Issue 13, p. 53) The fifth edition, FantastiCon V 2K, held from 14 through 16 July 2000, was Star Trek themed and was well represented by Star Trek cast and production staffers, old and new. Several staffers were awarded on the occasion. As to underscore the fondness Campbell had for Star Trek, even though the convention celebrated other franchises as well, he had christened the most prestigious FantastiCon Award, the "Gene Roddenberry Award". Unfortunately, upon the failing health of its founder, the convention has become defunct.
Campbell died on 28 April 2011 at the Motion Picture & Television Country Home and Hospital of the organization he held so dear to his heart, in Woodland Hills, California, following a lengthy illness. He was 87 years old. 
Star Trek interviews
- "William Campbell, From Naughty Boy to Nasty Klingon", Robert Greenberger, Starlog, issue 128, pp. 17-20
- "William Campbell", Star Trek: The Magazine Volume 1, Issue 13, May 2000, pp. 48-53