Do they add to a character's, well, character? Or do they just pile on the corniness?
Ha, who am I kidding. It's totally the former. Good ones, at least, which are the ones most widely known. For example, Bazinga, anyone? Sheldon Cooper's catchphrase not only enhances his character, but also serves as an inside joke for anyone who watches Big Bang Theory. Or, another one of my favorites, Gus, don't be a... a la Shawn Spencer. Five by five from Buffy the Vampire Slayer. The famous Gibbs slap (yes, a slap can be a catchphrase). Allons-y! a la the tenth Doctor.
These are just off the top of my head. (Apologies that they're all TV catchphrases, those are the most fresh in my mind. Book catchphrases work the same way, though--recognizable, quirky, unique, and oh yeah, fun.)
What are your favorite catchphrases?