Once you have splined your animation, added breakdown poses and taken it as far through pose to pose technique as you are happy with, you can now advance to taking straight ahead passes at individual body parts to make sure they are all animated with nice arcs and add overlapping action and follow through.
Two closely related techniques which help to render movement more realistically, and help to give the impression that characters follow the laws of physics. "Follow through" means that parts of a body should continue moving after the characters centre of gravity has come to a stop. These parts keep moving beyond the point where the character stopped and then come back to rest a fraction later. "Overlapping action" is the tendency for parts of the body to move at different rates.
Most natural action tends to follow an arched trajectory, and animation should adhere to this principle by following implied "arcs" for greater realism.
Adding secondary actions to the main action gives a scene more life, and can help to support the main action. A person walking can simultaneously swing their arms or keep them in their pockets, speak or whistle, or express emotions through facial expressions.