Everyone has their own workflow that they find comfortable and works for them. For me, this is what I usually do:
1. Get assigned a shot.
2. Research and study. Learn about the character, the required actions, what my character wants, where he came from and where he's going. If it's just action or is a creature, then I study the physiology of that creature, figure out how he should move, etc. As much as possible, it's good to find reference of a similar creature that exists in real life. (For example, if you're animating a dragon, you can study eagles and lions)
3. Do my thumbnails and video planning (so I know which poses will happen on which frames - at least roughly, and include breakdown poses). When possible, I'll show this planning (or video reference) to my animation director for feedback.
4. Then I simply recreate my thumbnail drawings in the computer, thinking of each pose as a "whole drawing," where I'll pose out the whole character and save a key on every single controller, even if it hasn't been moved. At this stage, I'm also exaggerating the timing of the motions and pushing the poses into something more dynamic and interesting, but keeping to the same basic body mechanics I've just studied.
5. At this point, you should be 80% finished, if you've done your planning correctly. (See my very first Tips & Tricks articles about this). This is when I'll show my shot again for more feedback.
6. If they like my blocking, then I start cleaning up my curves (making sure all my tangents are correct in the graph editor, removing any redundant keys, etc) and polishing the animation, doing the hands, feet, fingers, toes, tail, whatever else needs to be done.
7. Then I do the face.
8. Then I do the mouth.
9. Then I show it again and hope to hear the magic word "Final!"
- Shawn :)