Me and a friend of mine use to do this sort of sh*t all the time back in High School where we would do voice overs, inserting dialog that looked like words he was saying. The trick is to watch the video on mute, that way your mind is filling in the gaps based on lip movement, rather than being swayed by audio if it were un-muted. If the audio is running it makes it difficult to create original dialog.

Dude, you missed your calling.

Funny thing is I too lost an opportunity. In my late teens my cousin (when we were close friends) and I went to the drive-in and turned down the window speaker and we would throw out comments and dialog here and there (ala Mystery Science Theater 3000)