I just came across this video where it gives a quick breakdown of how the look of Paperman was achieved and how the 2D drawn lines were added using their own custom 2D painting software. The software seems to have a sort of tween feature between key frames, which looks interesting but still it means you wouldn’t get the movement in the linework unless you were to draw each frame, which at one part of the video it looks like they have done. The video is here.
