Straight Ahead
and Pose-to-Pose
These are the two methods for drawing animated scenes. Each has its own benefits according to the requirements of a scene and can be implemented in a combination of ways.
Straight Ahead
'Straight ahead' action is when animation drawings are done in a flow of one after another, without planning ahead with key frames as a guide.
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Shown here is an animation I sketched of Raymond Briggs' Snowman on a pad of Post-its, which was purely without thinking ahead of the next immediate drawing. I recall being in a kind of focused flow, remembering where I was in the middle of the action and looking to the next moment without needing more structure. It is often said that the 'straight ahead' technique is more fun to animate with as you are going with your instincts and drawing spontaneously. The looseness can provide an extra dose of energy to the animation.
The spirit of improvisation can create some real moments of magic that would never arise with more deliberate planning. The downside of this, however, is that the focus can be lost and the structure of the figures can wildly fluctuate. Therefore it's important to be selective about when to use 'straight ahead' action, as not to compromise the overall piece. Some things are by nature erratic i.e. fire, water, scrambling panic, and so rough unstructured animation could bring the exact feel required.
Pose-to-Pose
'Pose-to-pose' is the alternative, using key poses for characters and then inbetweening drawings to complete the scene. This method is necessary to keep of the scale of figures and anchor them to specific parts of the space/composition without drifting off track.
When aiming to sustain a consistent figure, it is important to be able to refer to key poses (shown here in black). Keys are the most necessary poses, similar to how comic strips or storyboards show the essential moments.
There's a stability that can only come from exact planning. If you know the destination of the action, it is therefore possible to measure subtle distances between poses and use arcs of movement for very smooth motion.
Here's the final version of this scene (from The Animator's Survival Kit by Richard Williams) which shows a clearly structured, smooth and moderately paced stroll to the whiteboard. If this were animated 'straight ahead', you would almost certainly lose some smoothness and consistency along the way.
There's not necessarily a correct process here, just advantages and disadvantages. In all likelihood, it's best to use a combination of the two, i.e. plan the key poses and play around with 'straight ahead' in the inbetweens. Try them both and see what works for you.