Basically one or more people spends every line using the lyrics from a song, and no one else notices.  The difficulty comes from finding a song well known enough that the lyrics can be used from, without the dialogue sounding like it was just shoehorned in around it.  If anyone thinks of any songs, feel free to post them. :D

Scene 1:

Jo : George at the bar he's a friend of mine
Sam : Is he? Hey George.
Bartender: Hey
Jo : He gives me my drinks for free
Bartender : No I don't... have you been stealing drinks
Jo : And he's quick with a joke
Bartender :I'm not joking, have you been stealing my drinks?
Jo : Or to light up your smoke
Bartender :Is that a euphemism?
Jo : But there's some place that he'd rather be.

Sam wanders off to man seated next to piano/elsewhere in bar

Sam : Sing us a song
Man : Wha?
Sam : You're the piano man?
Man : Yeah, what's it to you?
Sam : Sing us a song tonight
Man : Uh, what? I don’t sing.
Sam : Well we're all in the mood for a melody
Man : Oh really? Jo do you want a melody?
Jo : Beer's good for me. Thanks George…. You've got us feeling alright

 

Scene 2:
Done with Jamaican accents

1: So, have you got that album finished yet?
2: Here’s a little song I wrote
1: Excellent (takes song off 2)
2: You might want to sing it note for note
1: This is barely 4 chords!
2: Don't worry be happy
1: You’re not being paid to churn out garbage, I want to see some real music!
2: In every life we have some trouble
1: Oh there’s going to be trouble alright, the producer wanted the album finished by the end of the week!

2: When you worry you make it double
1: Double this crap and I’m just going to have twice as much rubbish.
2: Don't worry, be happy......

Other ideas:

The Killers - Somebody Told Me
"Well somebody told me that you have boyfriend, that looks like a girlfriend that I had in February from last year."
"I don't think I'm getting you"
"It's not confidential"
"I'm not worried about that, who's girlfriend was it again?"
"I've got potential"
"Wait, so if I've got this right, either you're saying that Trevor " (points to trevor, big muscly man who is listening in and looking menacing) "looks like a girl, or that you like to date manly women..."
(slowly edging away) "Ah, rushing, ah, rushing... rushing around"


Billy Joel - Piano Man

"Sorry mate, do you have the time?"
"It's nine o'clock on a Saturday."
"Ah, so the poker game will be starting soon?"
"The regular crowd shuffles in."
"Really? I haven't seen anyone"
"There's an old man sitting next to me making love to his tonic and gin"
"Well .. geez.. i'm enjoying it .. but that's a bit much"