Hi there, Nicolette!
I know in the newsletter articles I harped on planning darn near every month for a while, so many of you know that I believe strongly that planning is the key to excellent animation, particularly if you're in the first five or six years of animating, but probably for the rest of your career.
Planning, simply put, is the process of studying reference materials (pictures, found video reference, created video reference, observation in the real world, etc) in order to determine what moves in the body in order to create the proper body mechanics and emotion - and more importantly, WHY those things happen within the character.
You want to know exactly WHY the series of movements and events happens in the body before you ever start animating your scene. Ideally, if you're doing it correctly, you should know what key poses and breakdowns are going to be on what frames before you start animating.
In this way, *most* of your animation effort should be spent before you start saving keys on your character. By that point, you're just translating the performance you've already developed and the keys you've already determined and exaggerated into the computer on the frames you've already chosen.
Obviously, a big part of this planning process is determining the timing of these poses, and sure, many professionals still use old-school exposure sheets (sometimes also called x-sheets or dopesheets) as a quick way to clearly break their scene down, but many animators also simply use any piece of paper to do this. I know there are some software tools out there to aid in this as well, but personally I feel like it's easier to just do some thumbnails and jot down the correct frame number beneath the drawn pose.
Using the exposure sheet isn't really what matters, in my opinion. What matters is that you go to the effort to do as much planning as the scene requires within the deadlines you are given, and that you should know your intended timing before you start saving keys.
Sure, you might need to fudge the timing a bit once your poses are in the computer - and in fact, while I recommend exaggerating your poses in the thumbnail stage, I actually recommend (especially to newer animators) that the exaggeration of the timing happen in the computer rather than on the paper. It's easier to see, and easier to work with.
In other words, lets say you film your video reference over and over until you really nail a take that you think truly communicates the ideas and has the real emotion you are looking for. From that reference, you can find your key poses and breakdowns, and draw thumbnails of these poses. Beneath each pose, you can put the actual "real-life" frame that the pose happened on when you initially filmed it.
Now go through your thumbnails again, and work on exaggerating those poses into something even more exciting or emotional or heavy or communicative, but keep the timing frames the same.
This way, once you get those poses in the computer, at least your starting point is "true." This gives you a good foundation to start tweaking your timing (and poses when necessary) into something that matches what is in your head as the artist.
Hope that helps someone - thanks again for writing in!