Slim's Butt Monkey status as a prop. This is partly funny because his voice actor, David Hyde Pierce, plays an important role in the popular sitcom Frasier, and Slim has quite a lot of lines in the movie, but in character he always gets lousy parts.
P.T. Flea: "You're a walking stick. IT'S FUNNY! NOW GO!"
Francis provides a lot of funny with his short temper.
Slim:(holding Francis to one side, admonishing some flies) "That's no way to speak to a lady."
Francis: "I heard that, you twig!"
When Flik bursts in on the 'hospital' scene with the entire circus troupe there. It's the contrast between how Flik's so excited with his Eureka Moment, while Francis is (almost casually) making a statement that makes this so funny.
Flik: "Hopper's afraid of birds!"
Francis: "And I don't blame him!"
Just before that, Flik just sprints after Atta inadvertently gives him the bird idea.
"Pretend that that's a seed." "...but it's a rock." "I KNOW IT'S A ROCK! YOU THINK I DON'T KNOW A ROCK WHEN I SEE A ROCK?! I SPEND A LOTTA TIME AROUND ROCKS!"
Hopper telling Molt, "I swear... if I hadn't promised Mother on her deathbed that I wouldn't kill you... I would kill you!"
"Oh, you shoulda seen it! This bluejay — he has him halfway down his throat, okay? And Hopper — Hopper's kickin' and screamin', okay?..."
Immediately after Hopper's "I would kill you" line....
Molt: And believe me, nobody appreciates that more than I do!
Hopper: SHUT UP! I don't want to hear another word from you on this island. Do you understand me?
Hopper:(gets angry and shakes Molt) I SAID DO YOU UNDERSTAND ME?!