With motion capture, motion exists in the real world. It gets sampled and then applied to a computer character. With animation, the motion does not exist in the real world. It is constructed and only exists when the images are rapidly displayed, creating the illusion of motion. That describes the process.
There are several differences philosophically, however. Motion capture seeks to convince an audience through the accumulation of detail. When it falls short -- when it is criticized for looking like a waxworks -- it is due to insufficient detail in the motion. Therefore, the goal of motion capture is to increase detail to the point where it is indistinguishable from live action. As Ken Ralston says,
Trust me, when you’re sitting in dailies talking about Anthony Hopkins’s armpit hair for an hour, you know there’s a lot of effort that goes into every pore of the skin, into every eye and eyebrow. It’s a massive puzzle that has to be broken apart and then put back together again.By contrast, good animation seeks to eliminate unnecessary detail in order to arrive at the expressive essence of a motion. Motion capture concerns itself with addition; animation with subtraction.
If animators look down on motion capture technicians, it is because of the relationship that these two groups have with essence. If there is an essence in a motion captured performance, it comes from the actor. The job of the motion capture technicians is to accurately reproduce it. Any animated additions they make are a result of the technology's shortcomings or the director's change of heart. By contrast, an animator, if successful, creates the essence of the motion. This is no small distinction. There is a world of difference between reproduction and creation.
* * *(Spoilers below.)
By the time Beowulf kills Grendel, we know that lust leads him to lie. His account of the swimming race is verbally different from the evidence offered on screen. He claims to have lost due to sea monsters, but the visuals indicate that he lost due to a sexual dalliance with a mermaid. It is clear that Beowulf also lusts after the queen. Based on what the audience has already seen, it isn't necessary for Grendel's mother to promise Beowulf a crown. He would have sex with her for no other reason than her beauty.
When Beowulf returns from the cave, claiming to have killed Grendel's mother, he learns the truth from the king before the king commits suicide. Now Beowulf should understand that his lust has led him and his kingdom towards further disaster. Does Beowulf agonize over this? No. He doesn't return to Grendel's mother until after the kingdom is attacked. Does he rein in his lust as a result? No, he takes a mistress. There is no self-awareness in Beowulf's actions, only in his dreams of more monsters.
When Anthony Hopkins' king confronts Grendel, neither can kill the other. While they are enemies, their blood relationship complicates their situation and renders action impossible. When Beowulf confronts the dragon who is his son, he has no emotional conflict as to what he must do. The only emotion between Beowulf and the dragon is hatred.
Beowulf's lack of self-knowledge (not realizing how lust overwhelms his best interests and destroys his integrity) and lack of complexity (no qualms over the need to kill his only son) are what make him, and the resulting film, so empty-headed. The question is not whether Beowulf is successful as an example of motion capture; the question is whether he is a fully realized character. The answer to this question is not the level of detail reproduced; it is the nature of the character's essence.
Some may be tempted to blame the script by Neil Gaiman and Roger Avary, but I blame Robert Zemeckis. Any director who understands a scene can add subtext to it by directing the performances, choosing who the camera should be looking at and how a character should be reacting at any given moment. Beowulf is a character with little self-knowledge, even as he dies. The other characters in the film are cut from the same cloth. This is why the technique, even if flawless, is worthless in this case. Ultimately, the detail must lead us to some essence, but Beowulf hasn't any.