Arttista makes the most highly detailed figures for S scale, and their paint is a realistic dull finish rather than the glossy paint of other suppliers. They run about $6 each, so I used them only for very visible scenes.
I faced the same problem for filling passenger cars and decided on MTH part number 30-11043, a bag of unpainted seated figures that look like the populace from the 1920s--rather trim for O scale, but "modern large" for people today who have trouble fitting in economy class plane seats. However, at less than a quarter apiece, they are a steal. The bag hold 120 figures, 80 of whom are seated. There are only about 8 characters repeated 10 times, but different paint on them does make them different. (Painting flesh color on a male head makes him look bald next to his identical twin with dark or gray hair.) I used inexpensive acrylic paints from Michael's for the paint, and spent about 5-6 hours spread over a week painting one color at a time on all 80 seated figures. Black for some shoes, hair, pants, skirt; switch to red for some blouses, shoes, skirts, purses, scarves; switch to blue for pants, shirts, blouses, shoes, skirts; and so forth.
The painted figures are far from perfect, but in a passenger car no one can see any flaws. I used Palace Car seats for AM streamliners and heavyweights, and in some cases I had to cut off some feet to get them to fit. The AM Budd cars have nicely detailed interiors, and the MTH figures fit well into the seats.