Based on the Twelve Principles of Animation by Frank Thomas & Ollie Johnston
01 - Squash & Stretch.
When an object picks up speed, the weight or density can be displayed by stretching its shape. However, when it impacts another object, it will flatten out, or become squashed. Objects like basketballs, or bouncing balls will squash and stretch more than an object like a bowling ball. Most importantly, the volume must always stay the same. Meaning, if a the height of a falling object becomes taller when stretched, the width will decrease in size at the same rate.
02 - Anticipation.
In order to prepare the audience for an action that is about to be performed, a character or object should begin to move in anticipation of the action. This tells the audience where they should be looking, and gives them a clue as to what type of action is about to take place. A character such as a baseball player might bend his knees and wiggle his bat before swinging. Without any preparation, movements can seem abrupt or unrealistic.
03 - Staging.
The presentation of any idea so that it is completely and unmistakably clear. Just as in theatre or film, using staging to direct the audience's attention conveys what is of greatest importance with regard to what is being shown. If one character is wearing a party hat behind a cake with lit candles, it becomes apparent that it is their birthday. Through staging, the audience is allowed to come to this conclusion on their own, without having any prior context.
04 - Straight Ahead & Pose to Pose
There are two different methods through which a movement or action is animated. Straight ahead animation involves drawing each frame one after another, from start to finish. However, pose to pose animation is done by drawing the beginning and end poses, and then filling in the frames between them. Straight ahead animation is ideal for things like fire, as it would be difficult to predict what the last frame might look like. Character movements are generally done with pose to pose animation, as it can help to retain the initial proportions.
05 - Follow Through & Overlapping Action
When the main body reaches the end of a movement, all other parts will continue to move in order to catch up. Different parts of the body tend to move at different rates. If a character swings back and forth, the head would take longer to reach its end pose than the body. However, the hair would take even longer than the head, due to the fact that it is attached to a part that is already following through and overlapping. This conveys inertia, and makes the movements feel more realistic.
06 - Slow In & Slow Out
A movement does not happen at a constant speed from start to finish. Objects take time to accelerate and slow down. As an action starts, there are more frames near the starting pose, a few in the middle, and more near the end pose. More frames near the beginning and end will make the object seem heavier, and fewer will make it seem lighter. When a car comes to a stop, it has to apply the brakes and slow down before eventually stopping. When a car begins to move, it has to accelerate out of the static pose.
07 - Arcs
With few exceptions, all actions tend to follow an arc, or slightly circular path. When a movement is arced it tends to feel more natural and has a better flow to it. When a dolphin jumps out of the water, it would not move in a straight line to the peak of the action. Rather, it would follow a path that is almost a half circle. This principle also applies to limbs when in motion. As they move, they rotate about the joint.
08 - Secondary Action
An additional action in a scene to reinforce the main movement and add more dimension. Secondary actions give the scene more life, and can be used to add personality or character to an otherwise dull animation. When a cat walks, the main movement is the legs, however, a bouncing tail, or blinking eyes create an additional layer of depth. Most importantly, the purpose of a secondary action is to emphasize the main action, rather than distract from it.
09 - Timing
If a movement occurs over a fewer amount of frames, the action will read as being faster. However, if the action takes more frames to reach the end pose, it will appear as if it is happening slower. Less frames can make an object seem lighter, while more would make it read as something that is heavy. When two balls are shown performing the same action, yet one takes half the frames to do so, it will appear as moving twice as fast.
10 - Exaggeration
An action that imitates reality perfectly can come across as static or dull. Exaggerated movements can have a greater impact on the viewer, and make the animation seem more lively. If characters are eating, exaggerated movements can make them seem hungrier, or more energetic and lively. Nearly every movement can be exaggerated in some way. It can make motions more convincing, but the appropriate amount of exaggeration is different for certain acts.
11 - Solid Drawing
This refers to giving an object volume and weight. Utilizing solid drawing techniques can make a character or object feel as though they exist within a three dimensional space. Solid drawing is one of the more difficult principles to master, and when done poorly, can distort the proportions the object or come across as unconvincing. When a skateboard does a kickflip, it rotates around the z-axis and shows perspectives of the object not originally shown.
12 - Appeal
When an animation has appeal, it means it has a quality of charm to it. Appeal can be equated to what is called charisma in an actor. Characters should be pleasing to look at or come across as likable. Symmetrical or baby-like proportions are a proven way of making the audience more attached to a character. To add appeal to an animation of a dog, it could be made to look like a puppy.