When I ask my students what they think good acting is, the first answer is usually "believable acting". But credibility is only one side of the story. Good acting is believable and interesting. In my opinion, these two attributes wholly define good acting. With this idea as an axiom, we will try to separately analyze what makes acting believable, and what makes it interesting.
I. Believable acting
In the life of an animator there are short and rare moments of true magic. Those moments are the reason I became an animator, and they are the reason I still am one. I'm talking about a moment in which you look at the animation you've just created, and suddenly you believe your own character. Suddenly it's alive, it's there in its own right. Those are the moments of believable acting.
Believable acting holds a great power over the viewers, because the character they're watching gets a sort of meaning. Every man has meaning to us - even if we don't always think about it: If a total stranger sitting next to you on the bus suddenly collapses, you will not be indifferent - because the very fact that he is a flash and blood human earns him that meaning. This is why we feel sorry when Bambi's mother dies: we believe her and we believe Bambi, and both of them mean something to us. On the other hand, the characters in South Park are anything but believable, which is why there's no problem killing Kenny in each chapter.
(This might be the right place to reemphasize that the animator is of course not solely responsible for contributing meaning to the characters - script has an important part in it too. This article, however, is dealing with animation).
Believable acting means that the audience feels that the character's actions are the result of its own inner motives, and not the animator's inner motives; that the character feels, thinks and reacts consistently according to its personality and mood. I emphasized the last sentence since it encapsulates many of the ingredients of convincing acting:
The aim here is not just to portray clear and defined feelings (happy, sad, etc.) but to look for a kind of inner feeling that we have in us all the time - maybe it can be called "consciousness". Try to "feel" your character when you create animation, not just move it around according to the principles of animation.
Your character shouldn't always act on immediate instincts. Look for opportunities to show thinking process, which leads to decision and action. It will enrich your animation with depth, complexity and believability.
Acting is actually more or less a series of reactions - the character reacts to its environment, to other characters, to stimulus. Every action must have a reason. Make sure you know what your character is reacting to, and that the reaction is reasonable (in other words: it's reasonable that this particular character will react in this particular way).
Retain a consistent attitude to your character's reactions. A shy character (small, timid movements) that unexpectedly acts in an extroverted way with no clear reason, will suffer great damage to its credibility.
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