Setting a rotation key for all your bones at a certain frame is OK to start with, but that is not the whole of it, only the beginning. That’s what you do with key poses, those that define a particular moment in the action that is the point to which previous motion flows and away from which following motions flow, on into the next key pose. But the way all the different parts get from key pose to key pose is what makes lifelike animation.
Here’s an analogy: key poses are like photos in an album. They can tell a story of an entire life, but not the entire story if that life. In between all the moments in pictures are many more moments where things happen in a much less linear fashion than the photos might imply, just like life.
Example – a standing jump. Basic key poses – 1) standing, 2) crouching, 3)extension, 4) rising, 5) peak, 6) falling 7) landing, 8) shock absorption, 9) recovery, 10) standing again.
From 1->2: hips get lower, arms swing back & maybe out, fully extended, upperbody tilts forward at the waist, knees flex, center of gravity shifts to keep everything in balance, and none of this happens at the same instant, it flows.
2->3: Arms swing up, legs push body up and stretches fully out, head goes back, spine arches backward, toes point downward.
3->4: Lower body starts forward swing at hips to set up landing, arms fall back to waist level, head levels out and spots landing point
4->5: legs draw up under body to gain peak height, feet set up for landing by ankle flexing back to horizontal or past, arms push down and extend, maybe with hands also pushing, head thrust forward some, hips swing legs/feet farther forward
5->6: legs stretch out to catch the fall, arms now behind torso, less extended, head comes down some, body now leaning “backward” in profile
6->7: legs flex to absorb shock of landing, arms swing forward to pull center of gravity over feet, body pitches forward at waist (inertia), head comes up from spotting the landing
7->8: Knees flex deep as weight of body pushes down, center of gravity moves forward a little past balance and arms swing back some to compensate, shoulders drop down due to inertia of upper body mass, legs start to push back up, center of gravity slides back over the feet, spine starts to straighten
8->9: Legs push back to standing position, spine straightens out, arms relax at sides, head levels out, center of gravity slides back past feet a bit as balance is restored, shoulders back up to normal resting position.
9->10: Center of gravity back to normal, body relaxes into standing idle pose.
And this is really only about half the detail the action needs. Many of the described motions are nearly but not exactly simultaneous, some must follow one another, some need to be only a frame separated in time to look right. In between all the key poses are lots and lots of interesting and critical motions that make the key poses work as part of a flow of action from start to finish. And this brief breakdown does not take into account such fine points as acceleration, or making the moves fit the character – is he/she fat, skinny, old, a kid, an athlete, a slob, a dancer, a klutz? That’s why there is no automated solution – only the animator can control these aspects of the motion.