I had worked out a way of using a combination of finely tuned filters on the crowd footage to make them resemble cartoons or painted characters, and then compositing footage of the other actors acting in front of a green screen. That way, I could make the green screen actors appear as regular people in a room full of cartoon people.
The frame-by-frame/rotoscope-type work would have mostly been keyframing changing matte shapes to block the filters from some parts of the scene, and would have been necessary when the cartoon people overlapped with the non-cartoon people while interacting with each other, and I mentioned to the potential client that the more of that they wanted the longer it would take.