The animator is responsible for drawing the key poses. These are also called extremes. The basic definition is: the farthest point that a part of the body moves before changing direction.
These poses are derived from the sketches the animator has made in thumbnail form.
There are two basic forms of animating: key and straight ahead. The two are essentially the same; you start out at drawing number 1 and progress on through the scene. If youre doing key animation, you leave some drawings out and draw the next extreme position and leave the inbetweens to a later time.
Straight ahead animation starts at drawing number 1 and then you do drawing number 2, then drawing number 3 and so on. The big difference is in the planning of the timing. With key animation, you can do all the extremes and then decide on the timing later. With straight ahead, you decide on the timing as you do each drawing.
Clay or puppet animation is an example of straight ahead. Because of the medium, the animation must be linear, theres no other choice. You cant go back and reposition the puppet because it wont match up exactly with the previous pose and a jerk in the action will take place.
In computer animation you can do either straight ahead or key or even both. You could straight ahead the scene but then go in and decide to modify a pose in the middle then make appropriate adjustments to the poses before and after. You can also alter the timing by adding in frames or taking some out.
One of the other things that differentiates straight ahead and key animation is that it takes longer to see the whole scene as a pencil test. If there a re 100 drawings all together in the scene and each one takes 30 minutes to draw, it will take 50 hours or 6 - 8 hour days to finish the scene completely. If youre animating straight ahead, it will take you the full 50 hours of drawing before you can see the whole scene as a complete pencil test. If you only draw the extreme keys as in our formula before, for the 100 drawings, youll have about 12 keys which will take you 6 hours of drawing, less than 1 day of drawing.
It would be very painful, after 6 solid days of drawing, to find out that the timing doesnt quite work in certain parts.