Post 2 of 4, Simulation. Refer to previous animated picture for reference.
While we can draw pictures, we still need to know what to draw.
In the above picture, Sadness (the character) is floating on a pillow cushion atop some molten lava. She's wearing a fuzzy sweater, and as she speaks, her face deforms in motion.
For something with complex behavior like lava, it may be easier to simulate it rather than have an artist animate its fluid advection behavior. Before we draw a picture of physical reality, we often need to simulate physical reality itself.
The sweater is made of cloth, which behaves somewhat like a dampened system of springs (where each sweater thread is a spring). That has to be simulated so it behaves like a sweater when Sadness is moving.
Sadness's hair is also a system of springs that absorbs kinetic energy from the movement of her head, and is influenced by wind.
The lava gives off light (which feeds into the light transport simulation). If the lava model was built from first principles, we might actually calculate the light using the amount of blackbody radiation emitted by molten rock.
This might be unnecessary though, if the shading artist simply decides on the emission color ahead of time.