I think Randy Quaid was just as funny as Chevy Chase. Their scenes together are pure gold. Couple of the lines that always crack me up -
"He's got a little bit a Mississippi leg hound in him. If the mood catches him right, he'll grab your leg and just go to town. You don't want him around if you're wearing short pants, if you know what I mean. Word of warning though, if he does lay into ya, it's best to just let 'im finish."
"That somethin' ain't it? She falls down a well, her eyes go crossed. She gets kicked by a mule. They go back to normal. I don't know."
"Don't go puttin' none of that stuff on my sled, Clark. You know that metal plate in my head? I had to have it replaced, cause every time Catherine revved up the microwave I'd piss my pants and forget who I was for a half hour or so."
I don't know how Chevy Chase kept it together in the scene outside when Eddie and family crashed the lighting ceremony, or that next scene in the house with Eddie and Clark in the living room by the Christmas tree. It looks like Randy Quaid is just ad libbing and just keeps going deeper and deeper. I keep hoping there is a long lost reel of film somewhere that eventually gets found and released. I bet there are some hilarious outtakes of those scenes. One of the funniest, most quotable movies ever made.