To get enough extras for the Battle at the Black Gate, a few hundred members of the New Zealand Army were brought in. They apparently were so enthusiastic during the battle scenes that they kept breaking the wooden swords and spears they were given.
Since John Rhys-Davies suffered constant rashes from wearing the Gimli make-up, the make-up department gave him the opportunity to throw his Gimli mask into the fire on his last day of pick-up photography. He didn't hesitate a moment to grab and burn it.
So unpleasant was Sir Peter Jackson's experience in dealing with Harvey Weinstein and Bob Weinstein when the film was being developed at Miramax, that their shared credit at the end of this film appears over a pencil sketch of a man fighting off two oversized trolls. After Harvey was indicted for sexual misconduct in 2018, Jackson retrospectively called him "a real bully" who ruled through "mafia thuggery", and who would have probably taken the project away from Jackson and given it to Quentin Tarantino. He also forbade Jackson from casting Mira Sorvino and Ashley Judd, who both later accused Weinstein of sexual assault.
The last shot of principal photography was when the newly-crowned Aragorn bows to the four Hobbits. Although Viggo Mortensen did not need to be on-set for that day, he nevertheless insisted on attending. He didn't have a crown (it wasn't necessary, he wasn't being filmed), so he fashioned one out of paper. With each successive take, the crown was becoming more ornate and sillier as crew members kept decorating it, so the four actors playing the Hobbits often had difficulty suppressing their giggles.
Andy Serkis and Elijah Wood were each given prop rings by Peter Jackson used in the movie. They both thought they had the only one.
Henry Mortensen: The first Orc that Aragorn is seen killing on the fields of Pelennor is played by Viggo Mortensen's son.
Royd Tolkien: (J.R.R. Tolkien's great-grandson) as a Gondorian Ranger handing weapons to his fellow soldiers when the orcs are invading Osgiliath.
Christian Rivers: The art director and storyboard artist appears as one of the Gondorian soldiers watching the beacon in Minas Tirith.
Howard Shore, Michael Semanick: Seen over Legolas's shoulder during the drinking game in the Golden Hall, in the Extended Edition DVD.
Peter Jackson: [children] Jackson's children Billy Jackson and Katherine Jackson appeared twice in this movie: in Gondor when the horsemen leave the city, and in the Hobbit wedding.