Jason Nesmith's in-universe Creator Breakdown at the beginning of the film, to see a jovial, if not arrogant, star who at first seems to embrace the fandom in contrast to their least enthusiastic cast-members, to have his positive outlook crumbled after listening to the gossip of two non-fans about the negative affects of the show's fandom and how it tainted their careers, which sadly revealed to be not completely unfounded, and leads him to lashed out at his fans and eventually succumbed to alcoholism at his own home, as he watched old episodes of his show to pathetically trying to find his comfort zone again. Jason was probably thinking I Wish It Were Real for his Galaxy Quest television fame to be really worthwhile and eventually becomes a Crowning Moment of Heartwarming and Crowning Moment of Awesome for him when he realizes how his show inspired an alien planet to turn it into real experience, thus making him as giddy as child when his dreams comes true as Jason gladly embrace his television role for real.
Jason explaining being forced to explain the true nature of Earth television and fiction to the naive Mathesar. It's like watching a slow, hard breakdown of a child discovering that there's no such thing as Santa (it even hits harder as Jason's actor played a Santa). He looks absolutely devastated.
Not to mention Mathesar's cheerful face as he tries desperately to hold on to his belief in the show, vainly attempting to counter Jason's statements about how the "ship" is actually only an inch long.
It's even painful how Jason finally manages to get through to Mathesar. As mentioned before, his species doesn't understand the concept of fiction and playing pretend (which is usually not malicious in intent) but through their interactions with creatures like Sarris they now understand the concept of lying. Jason tries to break it to him gently by saying that he's an actor and that they were merely pretending, but Mathesar doesn't understand, forcing Jason to use language the Thermian does understand: "We lied." Mathesar visibly flinches at the word as if he's been stabbed.
The entire scene when Sarris sneaks onto the bridge and starts killing everyone off. Even if you already know Jason's going to activate the Omega 13. It's still an amazingly horrific scene.