The standard approach to an animation project is, and pretty much always has been, to first draw the main moments of the action, and then draw the moments that fill in between the main moments. We call these main moments "keyframes" and the moments in between are called in-betweens or "tweens".
The drawing of tweens is the drudge work of animation. But that's what computers are for. They are built to do drudge work, and they do it fairly well.
For instance, consider this plucky bug who is about to get smashed.
From the basic drawing above, we create the next "key moment" of our drama - the realization he's about to get whacked. This becomes our second keyframe.
Then we continue to dupe and modify until we get all the "key moments" covered.
Now, instead of drawing all the in-between frames or tweens by hand, we can use Illustrator's blend tool to do it for us. All we need to know is how many frames we want between each keyframe, and we decide that by deciding how fast we want each segment of the sequence to run.
When we get all done, we have what looks like about 24 individual frames which we "drew" in under 5 minutes.
You can see by looking at the layers palette that the original keyframe layers are empty - as well as some other layers. That's because the blend tool compiles the objects into a single layer, and when we break that blend object into individual layers, we pull them out so that they stand as layers on their own. This leaves those container layers empty.
A good practice is to move the empty keyframe layers (assuming you've designated them as keyframe layers) into positions below the frames which the actual keyframe objects now occupy. This way, you can find them easily.
When you export the .swf file, you get something like this:
This is a fully rendered (except for the ankle), ready to go piece of animation that took less than 15 minutes to create.
In the projects for his lesson, we're going to look at some basic ways of handling tweens in Illustrator using the blend tool.