There's a lot of different workflows in animation. I want to talk a bit about the main ones and what they're good for and not so good for. First of all let me say that most animators tend to adopt at least one method and then personalize it, and although I won't say that one workflow is better than another, I can say what I find from my experience to be the pros and cons of each one. Most animators will know the basic concepts here and there might not be anything new, but hopefully some people will find something about it a bit interesting.
Do now/fix later vs. planning.
All animation should involve some planning but there is also something to be said for the trial and error approach of just making the keyframes and then deleting/moving/changing them as you see fit. It certainly helps you understand how and why things work in animation. The problem is that, especially for an inexperienced animator, you're going to make a lot of mistakes and it's going to be gradually more difficult to change your animation if something doesn't work, that could be spotted in a progressively more detailed planning process. Planning things well out can help a lot on the quality, but in many cases, with two equally experienced animators, the guy jumping in feet first and fixing things as he sees them, will complete the shot much quicker than the one planning it in great detail. In a heavily directed setting, with a lot of changes and contemplation, it will be the other way around.
Straight ahead: The favourite method of the do now/fix later animator. Possibly the quickest workflow from shot on desk to shot out the door IF the following three criteria are met: 1. You're a decent animator. 2. You know what you/the client/director wants. 3. There's minimal changes once you're done. "All" it involves is knowing how the animation should look like before you start :) No mean feat. But it works great in many circumstances. The motion is fluid. You create follow throughs and breakdowns as you go. In many cases the director/client/whatever have told me 'great job' after just a single pass through with this method. On the desk and out the door the same day. I recently started the lip synch/facial animation part of a 25 second acting shot using this method and it was complete just after lunch the same day, with the director commenting on how good the lip synch was! Now the other side of the coin is that had it not worked out, I would have had a lot of useless animation on my hands to clean up. And since the director/lead animator had no input other than the brief at the start, it could've ended up being quite a wasted bit of energy. Arguably, if you can work so much quicker this way than pose to pose that you could afford doing multiple attempts, or if the changes are small, it could work out fine. In the end, this method is most suitable for short time frame work with little or no changes needed. And it's a very suitable method for boshing shots out for things like kids tv as well as short and sweet animations. When I work freelance doing shots and ads and there is not neccessarily a director or lead animator to turn around to for opinion and direction, this is often my method of choice because I can burn through the shot at a quick pace and the client will have something 'complete' to address for the next day.
Pose to pose: There are several slightly different approaches to this, but the main goal is to get a good sense of how the whole shot will be early on. Lay down the key/golden/storytelling poses first. Straight away both you or anyone else working on it can see what is working and what is not for the whole shot. Many use stepped keys for this so they can focus only on the poses. Once this is done, I tend to progress to finding out how long each main pose should be held for, by setting all my keys to linear and copying the pose for the entire hirearchy to a new frame in between it and the next pose, and after that, start creating breakdowns, accents etc in between each pose.
The main benefit of this method is quite simply that it's possible to review what works and what doesn't very early on, before you lay in all that work of getting the motion looking nice. You can pass the shot to another animator quite easily, or he can work on a different character in the same scene, reacting to your key poses while you continiue to refine the motion. The main drawback with the method is that since you're not seeing exactly how your motion is going to be throughout the process, one typically finds oneself spending more time tweaking curves and working with the inbetweens, and overlaps, follow throughs etc are more difficult to get fluid (you can get a lot for free here in straight ahead animation). In the case of long form, heavily directed and/or multi animator shots this is definately the method of choice though, as the benefits most often far outweigh the drawbacks for these types of animation. 'Can we bring the character over to the left of the screen as he's doing that?' could be a killer question in straight ahead animation, but could be relatively easily spotted and changed early on in the process of pose to pose animation.