The Best Line Wasn't In the Movie
It was on the set.
The director famously had trouble with Marlon Brando making this movie, but he also had trouble with Richard Boone. Brando and Boone were both intense, opionated men.
Evidently, the director asked Boone to do something a partiular way in a scene, and Boone gruffly replied:
"What? OK, I'll do it. It makes as much sense as a rat f---king a grapefruit, but I'll do it."
Soon thereafter, the director quit the film and, at Brando's request, Boone took over the direction -- and started fighting with Brando as his director, too.