Before you put down the first line of animation, take the time to know every element of your story. That doesn't just mean the actions your character takes. You want to know why he takes the actions, how he takes them, how the rest of the world responds to his actions...and most vital of all, how he then reacts to the world before entering into a new set of actions.
It is easy to forget that while a character's assertive actions drive the story forward, the reactions are what define the motives, desires, hopes, fears and loves that give the story meaning. If you can create a narrative in which every action has been given meaning by a constant, subtle array of reactions then you will have mastered the heart of character animation.
Tell a story. Know the actions...and know the reactions.
Storyboards and keyframes are basic tools of the animator. A storyboard is a version of the story told in visual frames, much like those in a comic book. The intention is to break the story down into the most vital visual images including the angles, lighting, action, mood and so on. A detailed storyboard can present the story in a manner similar to that of a final film, whether live-action or animated.
Keyframes are similar to individual blocks of a storyboard, but are entirely concerned with stages of motion. A series of keyframes produce the basic stages of an arc of movement. Consider a looped animation of a boy skipping: there will be a keyframe of the boy at the first lift of the skip, a keyframe of the boy as he launches, with the swing of his arms, a keyframe of the top of the skip, with arms fully extended, feet free of the ground, body arched. Then will come a keyframe of the descent as the arms fall, swing, shift. These keyframes will be gone through a second time to deal with the alternation from left foot to right, from left arm swinging up and forward to right arm swinging up and forward.
The keyframes provide the landmarks of traditional animation. The in between frames are drawn separately, leading up to the keyframes and out of them, creating a smooth sense of action.
Using these two tools, you can create a stable understanding of the flow of action and reaction throughout the animation, testing the "feel" of character behavior and refining the appeal and dramatic meaning of the choices you are making.
Creating your keyframes can be a challenge when you are new to drawing and animation. You need to track a three-dimensional form on a two-dimensional plane and keep all the relationships straight. Two simple approaches can help you keep the work from tying your brain in knots.
The first is the method of thinking of a figure as a series of circles and ovals and squares strung on long lines that represent the major lines of the body: legs and arms, spine, the line between the two hips. By breaking the figure down into these rough elements, you can quickly sketch out the proportions and the placement.
The second tip is effective: points and joints. The main joints of the body, and certain key points, are stable landmarks you can track in motion and which can help keep the rest of the body in alignment. Use a dot to stand for a knee, an ankle, a wrist and plot them along the arcs of motion as you create your keyframes. Don't worry at first about how the entire arm and all the fingers are moving: start with the wrist, the elbow and the shoulder. Similarly, when creating a closeup of a face in action as emotion plays across it, use the points of eyes, nose and chin to gauge the movement of the whole and provide a relatively stable base for movement.
Use circles and lines to establish the form of a character. Use points and joints as the primary elements of motion when building keyframes.
When a character isn't "in motion" what is he doing? In animation, as on stage, utter stillness is deadly. A frozen figure stops seeming alive...ice isn't nice. Yet there are many times your character must be stable while other characters act. In that case you are looking for "business" and for small, natural shifts. If a character has stopped its main motion, is there a follow-through motion? A balancing? A slight turn to follow another character?
These natural motions and follow-through actions combined with "stage business," in which a character is kept busy (and in character) by performing minor tasks, habitual motions and reactions not only keeps the character alive on screen, it opens up a huge stage for the "reactions" that are as necessary as action. Never treat a time of inaction as dead time for your character. Such time is a superb opportunity to flesh a character out and strike a spark in the viewer.
Use follow-through motion, balance and readjustment and reactive motion to keep characters alive on the screen and fill out their personalities. Frozen characters die on screen and waste your precious time for character development.