When playing NPCs, don't just sit behind your GM screen and describe what they do. Get up and prance around the table, cackling and lunging at players and speaking with a foreign accent, while flinging your arms in the air. Give them a visual impression of the NPCs they're interacting with.

Just be careful to keep their movements within the scope of things that you can physically do without crashing into furniture or pulling a hamstring or whatever.

Transcript

Fozzie: Get on with it!
Mark: {playing piano and singing} When you get in trouble, And you don't know right from wrong,
Mark: Give a little whistle, give a little whistle.
Mark: ♫
[SFX]: tap tap tap {Mark dances}
Mark: {singing and dancing} When you meet temptation,
Mark: And the urge is very strong,
[SFX]: klik {Mark clicks his heels in the air}
Mark: Give a little whistle, give a little whistle.
Mark: ♫
Fozzie: Excellent! Take five.
Kermit: Awesome, that was!
Mark: Okay, seeya. {exits}
R2-D2: Well I can see why Corey didn't want to do that himself.
C-3PO: He might have done a better job though...
Luke: {returning} You're not gonna trick me into singing. And, uh... whatever that other thing was.
GM: Dancing! It was dancing!
C-3PO: If you say so.