Planning out animation isn't an easy process. In theory it is simple, but in practice there are many pitfalls for the unwary. One of the most common errors made by novice animators is to leave the facial expressions until last.
You might think to yourself "I'll save time by blocking out the main poses, and then work on the facial expressions later", but the danger with this approach is that you might never get around to doing the facial expressions at all, and your animation will be lifeless and flat.
One of the early lessons I learned at Blue Sky Studios on
"Robots" back in 2003 was how to block out the character's main facial expression from the very beginning, thus determining the main acting beats in the shot.
What is the character thinking and feeling? This is a choice you should make at the very start.