To give your NPCs unique and interesting personalities, have some of them adopt the mannerisms of player characters. The various things that PCs do can give you an endless source of inspiration.

Transcript

Lieutenant Renz: You're under arrest!
Chewbacca: On what charges?
Lieutenant Renz: Laying charges. And throwing charges.
Lieutenant Renz: Get that Wookiee-gram's plasma crossbow! I think it's a legendary!
Chewbacca: I dare say, if you damage the lacquerwork, you'll be sorry.
Leia: How did this happen? We're smarter than this.
Chewbacca: Apparently not.
Lieutenant Renz: Lemme compare to my DL-21.
Leia: Did you just get a feeling of déjà-vu?
Lieutenant Renz: Sweet! My DPS went up from 87 to 131!
Han: Hey! Who's playing this game?