An actor who once starred as the captain in a sci-fi show named Galaxy Quest. He's arrogant, seemingly a jerk to his costars, and is stuck in the past, clinging to the old glory days of the show, enabled by the devoted fans who love his showboating antics. At the beginning of the movie, at yet another convention, Nesmith is confronted with how pathetic he has become and drinks himself into a stupor. It's in this state that he encounters the aliens and unwittingly agrees to his first real mission.
Badass: All those years of rolling around a soundstage covered in dirt really paid off.
Jumped at the Call: Once the interstellar shakes wear off, he's thrilled that a bunch of real, actual aliens built a replica of the ship and consider the cast to be actual heroes.
Large Ham: Much to the annoyance of his fellow actors.
The Leader: His position on the show, and he eventually takes the position for real.
No Celebrities Were Harmed: To William Shatner, the show's main cast member who has an inflated ego and is shocked to discover how the rest of the cast dislikes him for it.
Took a Level in Kindness: Contrast his appearances at the beginning and at the end of the film; at the beginning he's unashamedly hogging the spotlight and gets disappointed when he doesn't get the fanfare he wants. At the end, he realizes that the crowd at the convention aren't paying his costars any attention, and gestures for them all to come up and share the spotlight as they deserve it.
Unnecessary Combat Roll: He does this on the away mission before any actual danger occurs, out of habit from doing it on the show. It doesn't actually help him at first, and even causes him to lose his weapon. It helps him kill Sarris in the end.
Another star of the show Galaxy Quest now trapped on the convention circuit. Gwen played the show's fanservice character and is resigned to the idea that she never gets taken seriously. On the show, she had superfluous job of repeating the computer's statements and repeating the orders of the captain to the computer, so when the aliens build a real version of the ship, she gets stuck with that job again. There's some indication that Gwen and Jason were once genuinely attracted to each other but it died quickly once Gwen figured out what kind of guy Jason really was.
Action Girl: Though not in the old show, much to her chagrin.
No Celebrities Were Harmed: To Nichelle Nichols in her role as a bridge officer, and Jeri Ryan in being a skilled actress trapped as a shameless Ms. Fanservice character (and unlike Ryan, never being able to break out of that role).
Lazarus is a "Mak'tar", a member of an alien species renowned for their intellect. He is deeply intelligent and has psionic abilities. Additionally, he has a non-standard weapon and a pretentious catchphrase: "By Grabthar's hammer, by the suns of Warvan, you shall be avenged!". A trained Shakespearean British actor, Alexander resents both his catchphrase and being typecast.
Tommy parodies an actor in the "boy wonder" role, who has aged considerably since he was on the show. His role as Laredo is essentially that of the pilot, but when he flies the real Protector out, it takes him most of the film (and carefully studying his old Galaxy Quest episodes) to remember how to do it right.
Doing It for the Art: This occurs in-universe as, due to the fact that he was a kid at the time, he naturally took his role on the show quite seriously, to the point of coming up with his own consistent method of how the ship's controls worked. This comes in handy when the Thermians recreate the ship from the episodes.
Chen is in charge of the engine room and the operator of the "digital conveyor" (a version of the Star Trek transporter). He acts in a detached manner and is completely unfazed by the strange and often disturbing events that occur.
Bunny-Ears Lawyer: He may be a little nuts or detached but he's good at what he does... eventually.
Fake Nationality: "Kwan's not even my real name!" In-universe, he's an actor who plays a guy who changed his name then played a guy who is fake. In the process, the real actor played a guy who played a guy and must become that guy even though he's fictional.
Heroic BSOD: He freaks out when he's asked to operate the "digital conveyor," which could kill Jason if he does it wrong.
An actor who played an extra in one episode of Galaxy Quest. He suffers from paranoia that he will end up in a Red Shirt's natural state during the film. The character is based on real-life Star Trek actor Guy Vardaman, who played an uncredited background character.
Adorkable: Nearly two decades after his cameo on the show, he's still nothing but proud of it and perfectly satisfied with this tiny blip of a career.
Ascended Extra: From no name redshirt to chief of security in-show, of course.
For the Evulz: He already had the information he needed and explicitly invokes this trope, albeit not by the name it has on this very site.
Thermian Commander:I have told you all I know... please... if you have any mercy, let me die.
Sarris:I shall kill you when I grow tired of the noises you make!
Dangerously Genre Savvy: In his first fight with the heroes, he opts to drive them into a minefield which causes them to be severely crippled. Later, he disguises himself as Fred with an image inducer, sneaks onto the ship and kills pretty much everyone except Guy. This is only undone by the Omega 13, which he had no way of knowing about.
Gallows Humor: Leading to possibly his only funny moment in the entire film.
Jason: Hey Sarris, how're you doing?
Sarris: (shows the head of his lieutenant mounted on a stick) Better than my lieutenant!
Genre Savvy: Being a real alien overlord, the actors of the show have a very difficult time dealing with him because he's actually competent and intelligent. Really, the only two mistakes he makes are assuming the actors and Thermians are too emotionally broken to stop the ship's self-destruct sequence, and not realizing the Protector was dragging a field of magnetic mines towards his ship. When he finally sees the "historical documents", Sarris immediately realizes what's happened and has a jolly laugh over it, then forces Jason to tell Mathesar the truth, knowing the revelation will break his spirit harder than anything Sarris himself could ever do.
Wrong Genre Savvy: As he's not aware that the heroes are actors, he assumes that they can tell him what the Omega 13 does, and when Jason says he doesn't know, he assumes he lying under the belief that no captain could be that clueless about his own ship.
Handicapped Badass: He's horribly cruel and very depraved, but the guy is a good straight-up fighter in spite of lacking one eye.
He presented the Thermians promises of mercy only to continue his genocide.
Even when he has nothing to gain from, he decides to self-destruct the Protector and have the air sucked out of the crew chambers the Thermians occupy.
Knight of Cerebus: After it turns out that he's actually a real threat and not something from a B-movie, the humor rating drops significantly whenever he's onscreen, and one of the only two jokes involving him is a bit of Gallows Humor where he reveals he decapitated his lieutenant.
The leader of the Thermians, though he steps back to let Jason lead. He and his other people are completely ignorant of any type of falsehood, including fictionalized entertainment, and thus they have no idea that Galaxy Quest was just a TV show.
Break the Cutie: When Jason finally tells him the truth, Mathesar is broken almost beyond the point of speech.
Mr. Exposition: By virtue of As You Know — Mathesar is eager to show the crew how perfectly they've replicated the Protector, thus explaining the ship's functions to the audience as well.