GuiltyGearXrd's Art Style : The X Factor Between 2D and 3D

This is an artistic choice with a variety of film precedents. Its not exactly the same thing, but if you watch this GDC talk about the way that Arc System Works uses 3d to simulate 2d animation, it gets some of the ideas across: [this video] Artists might want to produce a lower framerate just to make something look filmic (eg, 25 frames per second) or hand animated, but it can also be a deliberate stylistic choice for other reasons. Eg, the recentish Mad Max films used subtle undercranking to make action scenes feel more intense, and part of that effect is a more noticeable frames and I think there is a bit of that in the Spiderverse films too.

A very simple implementation of cell shading is to run the result of traditional lighting through a lookup table in the form of a 1D texture. Having runs and steps of solid color posterizes the smooth lighting input so that it resembles painted cell artwork. A very simple implementation of outlining is to draw the object a second time solid black, with the vertex positions extruded out in the direction of the vertex normals, and with backface culling flipped so that you see only internal faces. Those black, internal faces peeking out around the edge of the regular mesh form the outlines. That's a good starting point. Many iterations and improvements from there gets you to this: [this video]