In this type of classic animation, you draw, paint, scratch, or anything else directly onto the film that goes into the projector. Once again, the artist creates the movie one frame at a time. Here's an example: "Aria", from glorialoyola:
Some artists add direct animation to images already on the film. Here's "Whiz!", a very creative example from gazzookabazookaz.
Direct animation really lends itself to abstract animation, though McLaren put character into many of his drawings and their interaction with the movie's soundtrack.
A major part of why I like direct animation is that the movie projector is acting as a microscope. A 35mm movie starts out on a 24mm x 18mm (1 inch by 3/4 inch) frame, and gets magnified up to a forty foot screen - that's 480x! The tiny details of the film surface and the inks, colors, scratches and whatever else the animator has used get blown up really big. In Aria, above, you can see glorialoyola's fingerprints on some frames.
Direct animation can be very free and immediate. Or as much work as you want to make it - it's all up to you.