Great, long form article from Joe Berkowitz. The inimitable Bill Plympton tells you how to get it done:
Hand-drawn animation is something that I feel really strongly about. A Pixar movie may be really great, but it looks like it was drawn by a machine. It’s all perfect and all the corners are exact and the circles are exact and I miss the warmth of a human hand--the mistakes, the little touches that a human hand can add, the idiosyncrasies of the hand that makes something more personal and warmer and compelling.