So, I know the principle of Solid Drawing is important, drawing the circles and simple shapes that make up the character, but should I draw it on seperate passes before drawing the lines over it, draw the bones and muscles seperately, or any other tips?
If I'm animating a scene on your own, do I redraw the construction on the inbetweens?
How can I work at drawing without having to build up the muscles one at a time like this?
Are you doing this by hand or software?
You have to work in a way that suits you best. From what you're describing it sounds you lack confidence a bit, so what I would do is study anatomy and life drawing. That will give you a proper sense of where the muscles should go, where the weight lies, how a three-dimensional body fills a three-dimensional room, et cetera. Because insecurity about those aspects just stand in the way of creating. It can be a nagging thing, can't it?