My favourite bit of that movie (Rickman plays Sir Alexander Dane, a proud thespian whose once illustrious career has been destroyed by Galaxy Quest typecasting him into this humiliating role):
Jason Nesmith: You WILL go out there.
Sir Alexander Dane: I won't and nothing you say will make me.
Jason Nesmith: The Show Must Go On.
Sir Alexander Dane: ...Damn you.
(Jason is fighting the rock monster)
Jason: Alexander, you're supposed to be my adviser, advise me!
Alexander: Well, you're just going to have to figure out what it wants. What is its motivation?
Jason: It's a rock monster, it doesn't *have* any motivation!
Alexander: Well, there's your problem, Jason, you were never serious about the craft!
No, it's Leonard Nimoy. This is a Trek TOS satire, and he's the alien sidekick who hates his catchphrase "Live Long and Prosper" or, in this case, "By Grabthar's Hammer, you shall be avenged".
It's possibly worth pointing out in later life he seemed happier about it -his second autobiography was called "I am Spock" https://en.wikipedia.org/wiki/I_Am_Spock
It actually kinda shows in the new movie. I mean, I was happy to see the old cast back, but Ford didn't really seem to be feeling it a lot of the time.
But on the other hand, that might just be an oldish man not really feeling comfortable playing an immature character (that's part of Han's charm, really).
I'd wager his dislike of the Han Solo character had more to do with working with George Lucas.
When the elevator to the spaceport goes down and the ship is revealed, Rickman's character says "Oh, my God... it's real." It's such a little line but it encapsulates the whole spirit of the film. https://www.youtube.com/watch?v=RrYsg2RFL30
https://en.wikipedia.org/wiki/Galaxy_Quest