does anyone know of information about the shader approach used for this game?
Don’t have any first-hand knowlege, but it looks like a combination of traditional toon shader techniques (an artificial color ramp instead of a cosine falloff on the lights, an extruded outline with some z-buffer push) with textures that are painted to meld with the dark outlines. The hardest effect to explain is the hard outlines on non-exposed edges (the bridge of the dude’s nose, for example, or the line of the lady’s jaw): those might just be painted in or maybe they are somehow keyed to fade in only when they are not facing the light. In earlier iterations they definitely showed some of the typical toon+outline limitations, like hard outlines that swim as the geometry turns and the hard-coded offset of the extruded outline peeks through some parts of the model.