It was later revealed to Kirk and McCoy that the commandant was part of the Khitomer conspiracy and was ordered by GeneralChang to allow them to make an escape attempt so they would appear to have been killed while trying to escape. The commandant captured them escaping as planned, and was about to tell them who wanted them killed before executing them, when they were beamed aboard the USS Enterprise-A.
In the various instances of creating his designs, makeup artist Richard Snell often took it upon himself to establish a backstory for his creations, as he often found the actor fulfilling the role inquiring about why the character's look is a certain way. In this instance the backstory he created for the commandant's was "–you're basically no good anymore to [the Empire] as a true fighter, maybe as a strategist. Now you have this horrible disfiguring scar and even though it's debilitating, you're making it work for you. We're going to make it ooze a little and you're going to use it over your prisoners as a psychological tool because you're using your mind." Snell continues, "I went into a great deal of character development for all the characters I did. The commandant is a little Napoleon. He rules with an iron fist. But in Klingon life, small people don't usually get ahead. Klingons are a huge race, particularly the ones that were used on the ice planet. We used even bigger than normal Klingons; these were the baddest of the bad. These were the guards at the worst prison in the galaxy, holding onto prisoners that were alien and all sort of different capabilities. They were absolutely gigantic, yet their commandant was half their size. How did a man like that get to be in that kind of power and respect given their background? So in terms of makeup, we put a huge scar through his eye and blended off the prosthetic down below the eye. It's a dead eye oozing stuff, and it's just kind of creepy to look at. In my mind, this character knows it. He knows that's it's difficult to look at an eye that has seepage and he puts it right in your face as a psychological tool." (Charting the Undiscovered Country: The Making of Trek VI, p. 92-93)
The first guard used a painstik on Kolos and was promptly attacked by Archer in retaliation. After Archer was subdued, the guard threatened to have Archer spend the night on the surface where no one survives.
The second guard shocked Archer with a painstik after Archer had attacked the first guard. (ENT: "Judgment")
The first Klingon was played by Danny Kolker who was credited as "Klingon Guard". The second Klingon was played by uncredited stunt actor Michael Papajohn. 
This alien was played by an uncredited Dennis Ott. His scene was cut from the version of the film aired for television.
To create the distinguishing changes in his skin color, "Ed French employed fluorescent makeup so ultraviolet light could trigger changes in coloration. This required the assistance of technicians from the Wildfire company," who used the same technique originally used for the Tarchannen III species in TNG: "Identity Crisis". "Nicholas Meyer learned the possibilities of the effects and suggested they increase the intensity of the ultraviolet light to communicate pain." It was also indicated that this was "an alien bully who has lost more fights than he's won" [...] as "was indicated by having the creature's horns look squashed." (Trek: The Unauthorized Story of the Movies, p. 176)
This humanoidprisoner was brought to the surface of Rura Penthe to demonstrate that no one could survive there. He was barely clothed and frozen within minutes.
The commandant told the arriving prisoners, including James T. Kirk and Leonard McCoy, that they should work well so they would be treated well, and if they would work badly they would suffer the same fate. (Star Trek VI: The Undiscovered Country)
This prisoner froze to death on the surface of Rura Penthe, when he was brought to the surface as punishment in 2293. His corpse was seen by Kirk, McCoy and Maria when they escaped the prison camp. (Star Trek VI: The Undiscovered Country)
The frozen Klingon was a dummy made to resemble makeup artist Edward French. French attached a Klingon headpiece to his own head and made a cast of himself with a tortured expression on his face as the foundation the prisoner's appearance. (Cinefex 49, February 1992, p. 44)
J.J. Abrams said the masks allowed them to avoid selecting Klingons that did or did not have the augment virus. According to him and the producers on the audio commentary for the deleted scenes, the Klingons come up to Nero in large numbers and throw him a chain to cuff himself because they're actually scared of him.
Around twenty Klingons were guarding the area the slaves were working. Most of them were armed with rifles and pistols and all of them were wearing helmets and coats. A group of six interrogated FourSquare and then thew some handcuffs to Nero to bring him to his interrogation. Two Klingon guards then escorted Nero through the tunnels of the plant and three were present when the interrogator questioned Nero. Nero later overwhelmed two Klingon guards when they brought him back into his cell. The Klingons in the deleted scenes were played by background performers including Terryl Daluz, Brandon Stacy, Scott Trimble, James D. Weston II, Cole Fritch, and Henry Jenkins and stunt actors T.J. Storm and Troy Brenna. For the wide shot in which the production used child actors, John Alan Bartlebaugh portrayed one of the Klingon guards.
The Klingon interrogator led the interrogation of Nero after he was brought in by the guards. He presented Nero his writings and drawings and told him he believe Nero is from the future. He also used a Centaurian slug on him. The interrogator was played by actor Victor Garber.
In a deleted scene, an alien named "4-Square" – who was named because he had four eyes – had kept papers of Nero's plans in his cell and was interrogated by the Klingon guards after when they found them in his cell. This alien was played by actor Tommy Germanovich and his make-up was created by Barney Burman and his company Proteus Make-up FX Team. It was given the name Quocch (β) in the Nero, Number Two comic book.