Everybody knows that, in fiction, Nobody Poops unless it's being played for humor. I have a vivid memory of reading a Star Trek novel (I'm pretty sure not based off of a TV series episode or one of the movies) where Captain Kirk is executing some important maneuver, a moment of high drama, and he's reflecting that what he really needs to do is pee really badly. It was something that stuck out to me as a child simply because you didn't often get books acknowledging that there were cases where that interrupted the action.
Unfortunately, I don't remember any of the actual action going on. I just remember that it was an incredibly dramatic moment that was kind of humorously being spoiled because Kirk was acknowledging biological urges and just how inconvenient they are. It would be nice if I could track the book down just so that I have some context to exorcise this from my head.
I am 99% certain it was Kirk, so that limits the scope a bit. I can't tell you exactly when I read the book, but it was probably in the early 1990s, as that's when I was old enough to read that particular shelf of my parents' reading material, and I went through it in pretty quick order. I might be able to try to track this down the next time I'm home, but my parents have also gotten rid of a lot the more worn paperbacks, so the book may no longer be there.