John Rhys-Davies is missing the end of his middle finger on his left hand due to a farming accident as a child. The make-up artists made artificial, gelatin fingertips for him to wear in the movies. Davies one day, cut the tip in half, put 'blood' in it and closed it up. He went over to Peter Jackson (unaware of the gelatin tip) and said, "Boss, I've had an accident, look what happened". Jackson saw a small cut, but Davies bent the tip back and it split open, gushing. Nice.
When filming the battle of Helm's Deep, the filmmakers used a computer program called "Massive," created by Steven Regelous. It uses Artificial Intelligence, AI, where each CG character has its own "mind." Huge battles can be created more realistically, because each CG character can be allocated a "side," and will then react/fight accordingly, in a variety of different styles (depending on the circumstances they find themselves in), rather than having to create and program each CG character individually. In the very first "Massive" battle test at WETA, soldiers were set to run until they found a target. This caused many of the soldiers to appear to be retreating the battle, after some were spawned facing the wrong direction.
Elijah Wood describes this as "spontaneous, violent love." Apparently Viggo Mortensen and some stunt men enjoyed greeting co-workers by 'head-butting' each other. One night he convinced stunt man Sala Baker to head-butt Orlando Bloom, who jokingly swears he saw 'white light', after their heads smashed together. A bright red mark on Bloom's forehead was not appreciated by the make-up artist the next day. Mortensen was amused because Bloom had the "perfect, pale Elven complexion." Appendices, Extended DVD.
In order to get the stunt Elves riled up enough to be on the offensive during certain scenes, the very aggressive stunt Uruk-hai would nastily taunt and mock them incessantly on the set and even call them, "Cupcakes." It worked! Commentary, extended DVD.
When Aragorn, Legolas and Gimli run up to search the burnt orc corpses for Merry and Pippin in Rohan, Aragorn kicks the metal Orc helmet on the ground. This particular bit was reshot several times and it is the last take they show in the film because Aragorn's scream and following sigh are so realistic. Well, that is because Viggo Mortensen actually broke two of his toes, when he kicked the helmet so hard in that take.
Liv Tyler had filmed several scenes of her fighting in Helm's Deep. Eventually the filmmakers decided for her character not to show up at that time, and cut and edited her scenes out. For instance, she was also pulling Gimli and Aragorn up the rope back into the fortress.
In the Extended Edition, Merry and Pippin are fighting about the Ent-draught. As the fight ensues, they are swallowed up by a tree. This is an homage to Old Man Willow, a character from the book "Fellowship of the Ring" that didn't make it in to the film adaptation. The hobbits are captured by the tree, until Tom Bombadil (who is also not in the film version) comes and saves them. Treebeard saves Merry and Pippin in the movie, using many of Tom's lines from the book.
During post-production, one of the effects technicians had to transport the first effects shots to a special location, since their computer could not send them all the way to Peter Jackson. After storing them in his iPod, he walked out into the street and was targeted by two thugs. After some serious sprinting, he managed to reach a hotel and save WETA's visual effects from falling into criminal hands.
Jed Brophy plays the part of an Orc who has his head cut off by an Uruk at the Fangorn Forest, while they argue about eating the Hobbits. Ironically he also played one of the Rohan Warriors who rode into the same scene and killed the remaining UrukHai and Orcs. I too was a Rohan Rider in that shot and worked with Jed on the scene. The rider scene was actually shot a year after the Orc scene, even though they are only seconds apart in the film.
When Gollum is catching a fish in the river, Andy Serkis had to act out the scene first, so the computer animators has something to work with. However, when they got to the river, it had been snowing and the whole river had turned to ice. The crew had four hours to defrost the river and get rid of the snow so the scene could be shot. They managed to do it, but Serkis still had to perform the scene in near-freezing water with so many thermal suits, etc, that he could hardly move.
In Faramir's dream, Boromir is in the boat, with water around him. The color of the water is pink, because Sean Bean's (Boromir) shirt was leaking dye into the water. Since it kept occurring in the retakes too, Peter Jackson decided to just leave it in, figuring it was a "symbolic blood-type effect."
Many of the Wild Men in the film were portrayed by actual groups of bikers, motorcycling around New Zealand at the time. Commentary, extended DVD.
While they were waiting between takes on set, the extras who portrayed the Uruk-hai at Helm's Deep started chanting, singing and tapping their spears on the ground, to the beat of their singing (probably due to the fact that many of the extras were Maori, the native people of New Zealand, and the chanting was a Maori haka). This is how the idea evolved for Peter Jackson to use it in the movie. It developed into the dramatic piece of the chanting and pounding of spears, by the Uruk-hai, at the stand off in Helm's Deep. As heard on the Appendices DVD.