(written from a Production point of view)
John Garman "J.G." Hertzler (born 18 March 1950; age 70) is an American actor from Savannah, Georgia, who has portrayed many different roles on several Star Trek series. He is one of only five actors to play seven or more different characters in the Star Trek franchise, the others being Jeffrey Combs, Randy Oglesby, Vaughn Armstrong and Thomas Kopache. Hertzler's best-known role is Martok, whom he played for four seasons on I AM ERROR. He also wrote the continuation of his character's story in The Left Hand of Destiny novel diptych with Jeffrey Lang.
In addition, Hertzler has provided voice-over roles on several Star Trek computer games, including Star Trek: Armada and Star Trek: Armada II as the voice of Martok. Hertzler also played the role of Koval in the independent production Star Trek: Of Gods and Men. In 2007, it was announced that Hertzler would play Harry Mudd in a forthcoming edition of Star Trek: New Voyages, which he would also direct.
In a special feature on the DS9 7th Season DVD set (disc 7), Hertzler remarks, "You know, the thing about Star Trek is they use more theater people, Shakespearean-trained (I've done a lot of Shakespeare – most of the Klingons have done a lot of Shakespeare) and that's something that Star Trek was unique. Is unique. They tend to go with people who can operate in a strangely heightened reality and somehow make it as close to reality as you can. That's sci-fi, you know, that's what you need."
Hertzler was interviewed for the DS9 documentary What We Left Behind.
Hertzler made his film debut in the 1978 independent horror film The Redeemer: Son of Satan!, which starred Jeanetta Arnette. He then appeared in the Academy Award-nominated film …And Justice for All, along with Keith Andes and Robert Symonds. In both the 1970s and the 1980s, however, he worked primarily on stage, including a stint on Broadway in an adaptation of the Greek tragedy The Bacchae.
One of Hertzler's first major television roles was a guest spot in the 1990 Quantum Leap episode "Sea Bride – June 3, 1954", where he appeared with Scott Bakula and Dean Stockwell. Hertzler then became a regular on the 1990s series Zorro, playing Alcalde Ignacio de Soto during the show's third and fourth seasons. Duncan Regehr was the star of this series, and Henry Darrow was also a regular.
Hertzler has since guest-starred on such shows as The Adventures of Brisco Country, Jr. (in an episode directed by Kim Manners and co-starring Morgan Woodward and David Youse), Diagnosis Murder (directed by Leo Penn), Dr. Quinn, Medicine Woman (with Chad Allen, Leland Orser, and Gregory Sierra), Seinfeld (starring Jason Alexander), and Lois & Clark: The New Adventures of Superman (with K Callan and Teri Hatcher). In 2000, Hertzler and Jeff Corey appeared as council members in an episode of Charmed. Hertzler also guest-starred on the science fiction series Roswell in an episode written by Ronald D. Moore; William Sadler was a regular on this series. Later that year, Herzler worked with Lee Meriwether, Keith Szarabajka, and Ray Walston on an episode of Touched by an Angel.
He had a recurring role as a gallery owner on the HBO series Six Feet Under, during which time he worked with Joanna Cassidy, James Cromwell, Ann Cusack, Matt Malloy, Chris Pine, Anne Ramsay, and Jeff Yagher. His other TV credits include the role of Black Dog in the 1994 family-oriented fantasy TV movie Treasure Island: The Adventure Begins, in which Anthony Zerbe portrayed Long John Silver and Shannon Cochran played the mother of the young protagonist. Hertzler also appeared in the 1996 mini-series Innocent Victims, along with Sam Anderson, Richard Fancy, Gregory Itzin, Don Keefer, Don McManus, Glenn Morshower, and Leon Russom. In 1999, Hertzler played director Ridley Scott in the biographical TV movie Pirates of Silicon Valley, which also featured Jeffrey Nordling and Marc Worden.
Hertzler has appeared in few feature films since the 1970s. His first film credit since 1979's …And Justice for All. was the 1998 direct-to-video horror thriller The Prophecy II, which also featured Elizabeth Dennehy and Tom Towles. In 2000, Hertzler acted in a short film called Jane along with Gary Combs. He has since appeared in two films starring Battlestar Galactica's Richard Hatch: 2005's The Great War of Magellan (which Hatch also wrote, produced, and directed) and InAlienable. The former movie also starred Brad Dourif and Richard Lynch, while the latter project was written by Walter Koenig and featured fellow Star Trek alumni Erick Avari, Gary Graham, Richard Herd, Andrew Koenig, Judy Levitt, Lisa LoCicero, Courtney Peldon, Jeff Rector, Alan Ruck, and Marina Sirtis.
In addition to his film and television work, Hertzler has supplied his voice to a number of video games, in addition to the Star Trek games mentioned above. He and his DS9 co-star Armin Shimerman both had voice-over roles in the 2007 computer game BioShock. More recently, Hertzler voiced Captain Benjamin Mathius in the computer game Dead Space, which also featured the voices of Scott MacDonald, Peter Mensah, Andy Milder, and the aforementioned Keith Szarabajka.
In November 2013, Hertzler, a Democrat was elected to the town board of Ulysses, New York, a suburb of Ithaca, twenty minutes south of a town called Romulus. He has been residing in Trumansburg, NY, and worked for a time as a theater and film department adjunct professor at Cornell University. 
On June 8, 2017, Hertzler announced his candidacy as a US House Representative for New York's 23rd congressional district in the 2018 elections. As part of his campaign appearances, he plans to act "in the persona of Mark Twain" to present his ideas "through the brilliant humorist for all ages." He stated that this was a tribute both to Twain, who resided once in Elmira, and to actor Hal Holbrook, who had portrayed Twain on stage for more than six decades.  Hertzler joined several fellow Trek alumni advocating against the administration of Donald Trump, including Whoopi Goldberg, Seth MacFarlane, Zachary Quinto, Patrick Stewart, and George Takei. 
- "The Way of the Warrior" (Changeling impostor)
- "Apocalypse Rising" (Changeling impostor)
- "In Purgatory's Shadow" (the real Martok; first of all subsequent installments)
- "By Inferno's Light"
- "Soldiers of the Empire"
- "Blaze of Glory"
- "Call to Arms"
- "A Time to Stand"
- "Sons and Daughters"
- "Favor the Bold"
- "Sacrifice of Angels"
- "You Are Cordially Invited"
- "Tears of the Prophets"
- "Image in the Sand"
- "Shadows and Symbols"
- "Treachery, Faith and the Great River"
- "Once More Unto the Breach"
- "The Emperor's New Cloak"
- "Strange Bedfellows"
- "The Changing Face of Evil"
- "When It Rains..."
- "Tacking Into the Wind"
- "The Dogs of War"
- "What You Leave Behind"
- Star Trek: Klingon (as Ler'at)
- Star Trek: Armada (as Martok)
- Star Trek: Armada II (as Martok)
- Star Trek: Elite Force II (as the Klingon, Lurok)
- Star Trek Online (as Martok)