Whether you are doing traditional hand-drawn cell animation -- like the animation in "Snow White" and other Disney classics -- or computer animation -- like the animation used in "Toy Story" -- the key is to keep things fluid. Keeping things fluid means understanding the importance of the keyframe. The keyframe is drawn by the head -- or master -- animator and represents a key moment within a scene. For example, if a character was to smile, there would be two keyframes: his expression right before and his expression right after. Once these two frames are drawn, another animator -- or team of animators -- will draw the stages in between. All of these frames are then combined to create the final product: a man smiling.
Always remember to keep the weight of a particular object or person in mind when animating to create a sense of reality. Even in non-realistic cartoons -- such as "Popeye" -- each object and character must be drawn with the appropriate weight. Showing weight can be accomplished by character expressions. For example, if Popeye were to lift a 100-pound dumbbell before eating his spinach, his legs may wobble, sweat may form around his head and his eyes may close, showing the struggle. When the weight of a particular object or person is not properly represented, the audience may become distracted, especially in the world of computer animation. Roger Ebert complained that the computer-animated Spider-Man in the 2002 Sam Raimi version lacked weight when he was swinging around New York, and looked more like a cartoon than a real person.
When it comes to both characters and the background within a scene, make sure to keep things alive. In other words, do not allow characters to stand around like statues and do not let the background rest with no sense of wind or action. If a character is stationary within a scene, allow their chest to expand and compress, creating the illusion of breathing. Keep your characters blinking, slightly adjusting their stance or looking around. For the background, have trees gently blowing in the wind, birds flying by or cars driving by. Even in the crudest of animation styles -- such as the style used in "South Park" -- the characters and backgrounds are always showing signs of life.
Similar to a comic book or a live-action film, animate your shots with a creative and engaging flair. Always make sure to animate the frames -- especially the keyframe -- as if you were directing real actors. Point of view (POV) shots, over-the-shoulder shots, panning shots, tracking shots, tilt shots and more should all be incorporated. The thrill of animation is you can place your characters anywhere and make them do anything, but the drama and/or comedy comes from the pacing of the action.