I once did use the general seperate every bodypart and reassemble to make different poses method (a sort of frankensteining, I suppose) when I first tried animating, but generally the results you'll get won't be as good.  I mean, there are certainly assets of poses you can and should reuse, especially heads to keep it consistent, but most new poses are going to need you to partially or wholly redraw the arms, legs, body, etc.  Generally, if I'm doing a running animation or something really dynamic like that I'll draw each frame from scratch--first just an outline with a blank color fill to test how well it animates, and then once I have something good there I'll clean up the lines and add details and shading.  For something simpler like an idle pose, I will generally select bodyparts and move them around, and then go back and clean up the damage I've done--redraw broken arm segments, smooth out newly formed jagged edges, etc.  In some cases I will nab an arm or leg from another pose, but generally I'll make some substantial edits to make it work smoothly.  Unfortunately, a lot of that relies on being able to quickly redraw/reshade those elements, which takes some practise.
Sorry not to go into more detail--it's a little difficult to describe, really, since it does also involve a pretty large amount of revision and C&C to get it to its final state.