Lord of the Rings actor Andy Serkis has revealed the lengths he had to go to for his role as Gollum, by performing each scene twice so the character could be computer-generated (CG).
Serkis said he gave the character "emotional connection"
Gollum, who features heavily in the second Rings epic The Two Towers, is a slimy creature who hinders the Hobbits' efforts in protecting the Ring from evil while they take it to be destroyed on Mount Doom.
Director Peter Jackson said it was vital for Serkis to act Gollum's scenes, so the computer could impose the CG image over that of the actor.
"Although it was computer-generated, Peter Jackson wanted an actor to lead the role," Serkis told BBC Radio 5's Phil Williams.
"You can't just have him be an animated creature with an animated voice on top because you wouldn't get that true emotional connection."
I haven't particularly done voice work before - this is totally extreme obviously
Serkis added that the intention had always been that the CG character would have its "psychology and physicality manifested by an actor".
The role required a lot of extra effort from Serkis, who spent a year in post-production doing "motion capture" wearing a special white suit covered in dots, which pinpointed the joints in his body for a computer.
The dots were fed through cameras to a computer with a CG image of Gollum, which Serkis watched as he re-shot the scenes to "hone" his performance.
Miranda Otto plays Eowyn
The actor, who said he had not previously specialised in voice work, had to provide a distinctive sound for Gollum, who has a split personality and talks in two very distinct ways.
"Gollum's voice is borne out of his mental state of mind, his schizophrenia and guilt at having stolen the ring," he said.
"He has two voices - Smeagal is the lighter naive victim who he was before he became Gollum, and Gollum is the harder, nastier creature he becomes. It came from the writing - fantastic script-writing," Serkis said.
He also explained that Gollum's face was based on his own muscle structure, which the computer used to create the creature's facial expressions.
Actress Miranda Otto, who is a newcomer to the Rings films, plays Eowyn the White Lady of Rohan, said her character was satisfying to play because she was "as good a fighter as the men".
But Eowyn got "even more ballsy" in the third film, she said, adding: "It was important to me she came across as a strong character and woman because in the book she very much is.
Legolas the elf [Orlando Bloom] fights for humanity
"It was a great character to play because on the outside she seems like the archetypal princess character but on the inside she's tough and strong."
Otto had to learn new skills for the film, including sword-fighting and horse-riding, but said her main battle scene had been "thrown together on the day", and that she was "lucky it actually turned out OK".
British actor Bernard Hill, who was Bafta-nominated for his portrayal of unemployed father Yosser Hughes in 1980s TV series Boys from the Blackstuff, said he preferred the Rings film to the book.
Hill, who plays Theoden, King of Rohan, said: "I found the book whimsical with too many tangents, and lost over-written and lost conclusions - things didn't resolve themselves."
He said the film script "made more sense to me", and that it was "a real distillation of the good elements of the film".
Frodo is played by Elijah Wood
Actor Karl Urban, who is also a newcomer to the film, admitted he found it nerve-wracking having to act in front of such major stars.
His first day of filming, playing Eomér the 18th King of Rohan, involved a battle scene with "300 guys in orc outfits and 500 guys on horseback".
"Right before my first shot I looked at the monitor and right behind Peter Jackson were Ian McKellen and Liv Tyler come to see what the new guy was all about.
"So I was battling away trying to keep the nerves under control as it was and it certainly didn't help the situation," he said.