I was approached by a Highwayman bandit and he said, "Hand over your gold or I'll gut you like a fish". I can't remember what movie that the line "gut you like a fish" was from. Does anyone here remember?

The Grinch.
I remember when he's making the stupid phone calls or someone calls him in his cave, he says something about gutting someone like a fish.

EDIT:
http://www.youtube.com/watch?v=U3eVzStZAjQ

Found a clip.

Hahaha that was good! I was thinking the line was from an older gangster movie. Is it possible that the Grinch borrowed that line from an old movie?

Could be, but that was really the only thing that came to mind when thinking of it.
Try googling it.

I'm pretty sure the killer in Scream also says it on the phone. But the expression doesn't really come from the movies at all, I think.

In the movie "The Soloist" where Robert Downey Jr. character, Nathaniel, yells, "If I ever see you again, I'll cut you! I'll gut you like a fish!" or something like that.

Yeah I remember that in Scary movie actually.

Turns out it's quite a common phrase.

Yes, thanks, y'all. It was a haha moment for me when he said it and what made it even funnier was the fact that (unlike in Oblivion) I said, "Do I look like I have any money?" and he said, "you know, you're right, now get out of here before I change my mind." It was just so funny!
Does anyone else have a funny NPC line to share?

I did have a funny NPC line to share, but then I took an arrow the the knee.

you did have a funny npc line to share but then your Grammar took an arrow to the knee.