Viggo Mortenson broke two of his toes while kicking the helmet after he thinks Pippin and Merry are dead. That take actually made it into the movie as Viggo turns his pain into a scream of despair that impressed Jackson. (Source)

The two sides of Gollum can be distinguished by his pupil size. The Gollum that is loyal to Frodo has larger pupils and the Gollum that is loyal to the Precious has smaller pupils. (Source)

When in Osgiliath, Sam says, "by rights, we shouldn't even be here." Intentional or not, this is ironic because, in the book, Sam and Frodo never traveled to Osgiliath. (Source)

How do you get the audio for 10,000 Uruk-hai? Jackson went to an England-New Zealand cricket match and put some Black Speech up on the big screen for the 2,500 fans to chant. (Source)

The poem Gollum recites in the Dead Marshes is an adaptation of an incantation that the barrow-wight uses on the hobbits in Tolkien's The Fellowship of the Ring: "Cold be heart and hand and bone/Cold be travellers far from home/They do not see what lies ahead/When sun has failed and moon is dead." (Source)