The principle of solid drawing means taking into account forms in three-dimensional space, giving them volume and weight. The animator needs to be a skilled draftsman and has to understand the basics of three-dimensional shapes, anatomy, weight, balance, light and shadow.
For the classical animator, this involved taking art classes and doing sketches from life. One thing in particular that old animation masters have always warned against was creating "twins" - characters whose left and right sides mirror each other, and look stiff or lifeless. Modern-day computer animators draw less because of the obvious advantages and facilities computers give them, yet their work benefits greatly from a basic understanding of animation principles, and this always carries through to 3D-CG animation as well.
One of the ways to impove solid drawing techniques is the line of action. It is a key ingredient to making your character’s poses look more dynamic. The position and posture of the characters in the scene can greatly effect the staging and composition for any animated scene. In addition, it can help to place the characters within the situation, making them part of their environment and the story.
Some ways to strengthen the pose (and overall composition) of the character is to create a nice silhouette, this is the overall shape of a pose. This shape should read clearly even if the pose were filled in black you would still be able to tell what the character is doing. The use of negative space and overlapping shapes for posing characters creates clear silhouettes, see the empty spaces between the arms and legs and major forms in these drawings:
A strong line of action through your character helps your poses "read", it makes them clear and understandable and gives them a distinct non-ambiguous direction. You can think of it as the back bone of a character or just as the invisible line that dictates how the body will move. This line should always be used in setting up a pose, it truly bring your drawings to life. Here's some Preston Blair examples with Tom & Jerry:
Here's some Sherm Cohen examples from Spongebob Squarepants:
If you study classical paintings, photography, comic books, and graphic design - you'll find that they all use this same principle. You need only to look at nature to find these lines of action:
Lines of action are not limited to characters alone, here's samples by Bill Peet on how invisible lines can be found in the overall layout of a well composed scene or illustration to create rhythm, flow, and a focal point within the shot:
Most storyboard artists and animators follow this method as a basic principle for planning out the acting and motion of the animated characters - their attitude and behaviors become expressed through their physical body. Body language comes in clusters of signals and postures, depending on the internal emotions and mental states, there is lots of non-verbal communication going on in every pose, see samples here.
Drawing, posing and staging your characters in an appealing and convincing ways can involve many different aspects: Composition, lighting, perspective, camera angle, negative space, visual balance, the attitude of the character, the scale of the character compared to other elements on screen. All while using the characters' mass, size and overall shape as part of its personality. In addition, a nice balance of curved and straight lines in a character's postures and actions must be achieved to compliment the acting intended - avoiding visual tangents, parallels, and twinning in the poses.
The attitude and behaviors of the character, facial expression, and body language are all things to consider when creating a set of poses. The stronger the poses the more appealing the animation will be. The Line of Action is this imaginary line extending through the main action of the figure and can help you in creating those strong poses, and better storytelling.
Here's an article by John K. where he specifies how animator Ed Love connects consecutive poses together. He doesn't always do an antic and an overshoot, and he doesn't time the connections the same way for each pose. What he does do, is control the whole sequence with a hierarchical structure of poses. Some poses and actions are more important than others, and he uses all the drawing and animation tools to keep your eye following the important parts of the action.
Here are the 2 poses we see and feel in the animation. They are holds. They are drawn with perfectly clear negative spaces, contrasts and lines of action. The action happening between them is visually obvious. Buzz stretches Woody up. The action is clear in just the still poses.
But to feel the the distance (or contrast) between them even stronger, Ed Love has created 2 more poses between them that caricature the held poses. He has made an anticipation pose and an overshoot pose. These 2 poses created more space between the extremes. That extra space gives the action more punch than if he had just inbetweened the 2 holds. (The farther you travel in the same amount of frames, the more punch the action has.)
Not every part of the second pose overshoots. The overshoot is focused on the main part of the action: Buzz' arm stretching Woody.
Focus of action gets to the final pose first. The rest catches up.