Before I go on, I wanted to drop a note about terminology here. I've used the terms "closely blocked" and "blocking plus" to describe a style of working that is antecedent to the approach I am proposing (or maybe that I will get around to proposing, as it seems like it is taking me many posts to describe the big picture!). To many animators, these concepts will be very familiar. But to some they'll be relatively new, and I've gotten the impression that not everyone knows what I'm talking about.
"Blocking" or "step-blocking" or "pose to pose" is the practice of creating key poses to "block out" the motion before turning it to spline interpolation, and then probably messing with it a whole bunch in the graph editor. It was introduced as a method of imposing some kind of structure on a shot from the beginning. If you started with splines from the get-go, you'd end up with something that needed a whole lot of iteration before it even got to the point where you could judge if it was working or not, which is a great way to animate yourself into a corner, or a padded cell.
(Not everyone necessarily agrees with me about this though--see comments to the previous post for a diametrically opposed point of view!).
So you'd first create a few step-keyed poses for the shot, and you could show it to the director and you'd be able to get some idea of whether you were going in the right direction. The problem is what happen next: you'd set all your keys to spline, and suddenly your nice, crisp blocking would turn into a horrific mess, which you then had to grovel through in the graph editor to fix into a presentable shot, a soul-killing process that turned bright-eyed young animators into hollowed-out shells. There were a few attempts made to come up with a systematic way to handle this problem, but they mostly resulted in motion that was pretty mechanical.
"Blocking plus," or "close blocking," or "pose and breakdown," or "really there isn't any agreed upon terminology" solves this issue by extending the blocking concept much further, allowing the animator to approach the shot as a series of step-keyed poses right up until final tweaking. Potentially you go down to using a pose on every other frame to every third frame, or even every frame for very fast motion. Creating so many poses sounds time-consuming, but if you have a breakdown tool it's actually a fairly fast process. If you have an onion skin tool it's even easier.
This allows you to do two important things. One is that you can watch something with a step-keyed pose on every other frame or so and actually understand the shot. It's not just the character popping between poses with no real idea of what the connective tissue will be: you can look at the shot and pretty much see the motion, and whoever has the authority in a given production can make effective judgements about it before it becomes difficult to edit.
The other is that, by nailing down the motion very closely before it's splined, you prevent interpolation from doing all that much violence to it. Now you can use the graph editor to tweak and finesse in a focused way, instead of trying to figure out how to get from pose A to pose B control by control. Blocking plus is, you might say, proto-interpolationless.
For a fascinating view of an animator becoming disenchanted with old pose to pose methods and discovering blocking plus, compare these two articles by Keith Lango. The second one was instrumental in my own discovery of blocking plus technique back in 2006.