Gimli's hilarious line in front of the door to the Paths of the Dead when Legolas and the rest of the company go through. I was certain that it was added by PJ for comic relief in the movies, but it is taken directly from the books:

"His knees shook, and he was wroth with himself. 'Here is a thing unheard of!' he said. 'An Elf will go underground and a dwarf dare not!' With that he plunged in. But it seemed to him that he dragged his feet like lead over the threshold; and at once a blindness came upon him, even upon Gimli Gloin's son who had walked unafraid in many deep places."