So how does this work? Drawn-on-film animators can use either blank (undeveloped) or black (developed) film in large or small sizes; which they use determines their technique, though many animators have made themselves famous for wildly experimental forays into drawn-on-film animation that deviate from the normal techniques.
The film reel is laid out across the work surface and fixed into place. The animator then works from frame to frame to create their image on each tiny, individual frame, adjusting it with each sequential frame to show the progression of motion. This requires a great deal of precision and talent, and also creates the recognizable scribbly, wobbly effect many associate with drawn-on-film animations. Using this method is far different from the in-betweening process that most traditional animators are accustomed to, and more closely resembles a flip book without the benefits of layered pages. Animators must judge by sight and skill the proper changes necessary to create a clean sequence of motion from one frame to the next.
When working with blank / undeveloped film stock, animators can treat the film just like a tiny piece of paper. They can draw anything they want, provided they use a medium that will affix to the film. The technique doesn't limit animators to just inks and paints, though. They can glue in anything from colored paper to pencil erasers - anything that floats their boat. Some have even been known to splice in existing film footage.
Another way to use blank / undeveloped film stock is in a darkroom, using a special setup with a small, focused light that's used to expose the film frames one at a time, generally with small items placed atop them. This creates a permanent impression of the objects on the frame. When the film is developed just like a typical photograph, the impression comes clear. This is almost like a combination of silhouette animation meets stop-motion animation, captured by manipulating film exposure.
Developed film presents an entirely new kind of canvas to work with, and a new set of tools and techniques. Etching and scratching onto the film aren't uncommon, and create a distinctive look conducive to certain animation art styles. Applying color to the black film can be a little harder, but layering it atop scratched areas or using tools like paint markers can ensure that the color stands out from the black backing. Some have even gone so far as to sand the surface of the film for a more diffuse effect, directly punch holes in it to allow light through, and use varying chemicals to directly affect the surface of the film.
One of the advantages of drawn-on-film animation is that it's relatively inexpensive, in that it doesn't require complex camera arrays, thousands of cels, or expensive software. A few simple drawing and etching tools, a roll of film, and a projector can be enough to let an animator explore their originality and play with a wholly unique medium. The simplicity of the format, too, forces animators to be more creative and innovative in storytelling through animated visuals. The medium leaves room for experimentation with everything from paints to film exposure to processing, and no two drawn-on-film animations are alike.