The stars of James Cameron’s Avatar put in such compelling performances as animated blue-skinned aliens that it raises many questions about how actors will work in the future and how that will be recognised.

Last week, I wrote that movie stars had lost ground in 2009, and that most of the year’s hit films weren’t hits because of their stars. This week, I am compelled to report on a new phenomenon which changes the nature of film acting itself: the emergence of seamless motion capture.

With his films Polar Express, Beowulf and Disney’s A Christmas Carol, Robert Zemeckis has, for some years, been capturing the performances of stars including Tom Hanks, Angelina Jolie and Jim Carrey and animating them in a computer. The effects range from impressive to creepy. And frankly, I always wondered why animate actors of that magnitude when you could shoot them in a live-action movie?

But watching Avatar at a James Cameron-hosted screening on the Fox lot last week, it was clear a new level of motion capture has been born. Some of his actors, namely Zoe Saldana, CCH Pounder and Wes Studi, spent months working on the film but only appear as Na’vi, 10-feet tall blue aliens with huge eyes, Shrek-like ears and a tail. Their facial features and voices are blended into the creatures and Cameron used their face and body movements as the basis for the characters. Sam Worthington and Sigourney Weaver also spend a large percentage of the film in Na’vi guise. Weaver, in particular, is unmistakeable.

The effect is startling. Working with Peter Jackson’s Weta Digital facility in New Zealand, the Avatar team has created lifelike digital characters which carry the story arc of the film. While watching, I was so engaged by the characters I constantly had to remind myself they were not human. It isn’t like watching Roddy McDowall in an ape suit or Gollum. It’s a whole new cinematic experience.

Cameron attributed the success of the process to his actors. “This was total performance,” he said. “That’s why Zoe and Sam had to train.”

I was also lucky enough to discuss motion capture with Peter Jackson, who was in Los Angeles to promote The Lovely Bones. Jackson explained that motion capture is “a way of moving. It has nothing to do with how the characters look, which is a design of the character. It’s terrific because instead of an animator and a computer animating a character frame by frame, motion capture allows a real actor to perform.”

“We were capturing body performance, hands, the entire facial performance,” added Cameron. “What the actors do in the moment is sacred. Once that moment is created, the technical processes that come afterwards cannot take away from it.They can only add to it.”

Jackson went on to elucidate that on the first of the Tintin movies, which he is producing and Steven Spielberg is directing, Jamie Bell, Andy Serkis and Daniel Craig are “bringing it to life” but the faces that will be seen on screen when the film is released at the end of 2011 will look like the characters from the pages of Hergé. “They are performing it as if they are doing the movie for real,” said Jackson. “And yet, what’s coming to life are characters that Hergé designed. They look as if he actually designed them himself.”

So not only are movie stars becoming less significant to blockbusters, but from now on these films will increasingly employ actors (and in the case of Tintin, some pretty famous ones) to ‘act’ the part. Yet their faces will never reach the screen. Quite how or whether awards bodies will recognise this new style of performance is a question that can’t yet be answered, but it’s certain that performance capture could become a key part of a film actor’s income in the future.