How to draw a 2D model for animation?

A friend of mine is a great cartoonist, but he has no experience drawing a character that must be rigged. I am new to Unity myself. So two newbies. If I am not mistaken you can't just take any cartoon and cut it into parts (on different layers). How does drawing a 2D character that will be animated differ from normal drawing?

As far as I can tell, it is especially the shouder that needs to be drawn as part of the arm on one layer. Like you see below. Question: all our characters will just stand, so no running etc. They will give a thumbs up and make other positive gestures with their hands. The arms will not be raised above shoulder level; no cheering etc. Is it enough if the cartoonist just draws the shoulders as part of the arms? Or are there other body parts that need special attention?

Thanks in advance!