One of the reasons (one of many) I really dig Firefly and Serenity is that the dialogue is very witty and sharp. There's nothing blase about the lines, never the cliche'ed crap that turns your stomach.
Example to illustrate.
It's the end of a very tumultuous day. Captain Malcom Reynolds and Dr. Simon Tam are in the cockpit.
Simon: I'm trying to put this as delicately as I can... How do I know you won't kill me in my sleep?
Mal: You don't know me, son, so let me explain this to you once: If I ever kill you, you'll be awake. You'll be facing me, and you'll be armed.
Simon: Are you always this sentimental?
Mal: I had a good day.
Simon: You had the Alliance on you, criminals and savages... Half the people on the ship have been shot or wounded including yourself, and you're harboring known fugitives.
Mal: Well, we're still flying.
Simon: That's not much.
Mal: It's enough.
