As Disney released more and more features, a distinct in-house animation style was developed. In particular, a technique called rotoscoping was frequently employed. Rotoscoping involves taking actual film footage and then drawing over each individual frame.
And so in the early days, when Disney worked on the likes of Snow White, Alice in Wonderland and others, the studio would actually film rudimentary live action versions of the scenes. Then the illustrators would take the frames and trace over them.
It’s a fascinating technique, and further down the line, Disney also figured out a way they could use it to cut costs. The took animated cells from previous films and then rotoscoped a new “skin” over the top. So essentially the studio was developing fresh characters and storylines from old designs.