Great passage from this Brad Bird Interview:
What do you think of the motion-capture films, like Beowulf?
Brad Bird: I think that Mo-Cap is a wonderful tool. Just look at how Peter Jackson used it, to see how affective it can be. I think the dirty little secret of Mo-Cap is that the little things that you really like have been massaged by animators. With Gollum, Andy Serkis did a wonderful job of physicalizing that character. I think that is brilliant. I also know that those scenes were massaged a lot, to look the way they do, by animators. Several of the most emotional moments with Gollum were key-framed. The animators looked at Andy, but they didn't use the Mo-Cap. They key-framed it. The scene that impressed me the most, where Frodo calls him Smigel, and Gollum goes, "What did you call me?" "Your real name." And Gollum thinks, "My name? My name?" And he starts to remember this part of himself that he has forgotten. You can see it in his eyes. And it's magnificent. I found out that it was entirely animated. It was not Mo-Cap. That's what people "don't" talk about. And I think it does a tremendous disservice to animators. There is nothing wrong with animation. Animators are not technicians. They are artists. They think about performance. I would implore actors to consider animators as brethren. We use different techniques, but we are just as much about the way someone stands, what they are thinking, are they hiding their thoughts? Is that depicted in their eyes? So, I feel like, if you don't muck with Mo-Cap, you don't get the performance from the actors, and you don't get the characteristics of anime. The best Mo-Cap I have seen has all been mucked with by animators. Much the same way the best roto-scope in Disney's time was mucked with. I'm not against Mo-Cap. But I think it has limitations if you don't mess with it.