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Tear Jerker: Galaxy Quest
Quellek's death always starts the waterworks. This innocent alien fan boy gets abruptly shot just after helping his idol save his crew, and has a few last words with him that should be narmful, but most certainly are not.
Jason explaining the true nature of Earth television and fiction to the naive Malthezar. It's like watching a slow, hard breakdown of a child discovering that there's no such thing as Santa. He looks absolutely devastated.
The varying looks of guilt on the other actors faces hurt too, even Guy looked ashamed.
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.