There are two ways of animating in 2D. The pose to pose method and the straight-ahead method.
The pose to pose method is blocking in all the actions that are going to happen overtime, like checkpoints through the over-arching action. Straight-ahead is going one frame at a time just relying on the earlier frame for animation, largely not knowing the direction one is going in.
The method that is most used by animators is a hybrid one. Where checkpoints (keys) are made but are not set in stone. This way one gets the organic flow of straight-ahead and at the same time is not entirely blind to where one is going. Result is, beautiful, lively animation that sticks to the available time.
This goal - spontaneity relationship is as necessary in life as in animation. Goals or checkpoints are important but not at the cost of sucking out novelty in living and exploring possibilities. There always has to be a small amount of flexibility in our goals so as to allow for magic to happen when it comes out of spontaneity. One can plan for the good things, great things can’t be planned, they have to be allowed into our lives by making more room for them.