Now that you have sets and surfaces, you need to RIG them. RIGGING means one thing in modelling tools and it means something completely different when you build sets that are large enough for people to walk around on.
Either way, rigging must still be created. So let's go back to the idea that rigging is for 3-D characters, in which case what you would have is a mesh. A MESH is attached to the important points of the character. For example, if there is a running stick figure, there must be a way of putting some sort of tracing on this character. If you can figure out where his joints are, you can move him in time and space.
If you can use this mesh rigging with a 2-dimensional character, then you can also rig a 3-dimensional character. And then of course, people who are really good at it would just start with the 3-dimensional character and work their way into the center of motion, the center of gravity, and apply the 3-D mesh just by using the tools.
Rigging is really important, because you need to understand that the character will take up space. If I were using a tool that would allow us to view this character in a 3-D space, then the mesh describes how this character fills the space. In which case, I am not spending time on the entire animation, I am spending time moving the mesh in space.
If you thought of the rigging as being a skeleton or an armature, then the mesh is the next level of the animation of the actor, and you can take these character components and move them around without worrying about armor or texture.
How you move the components is partially separate from their physical appearance. This is what allows you to perform tasks like paint the character later.
Consequently, in 2-D you have the actor and the animation. In 3-D you have an actor, the animation, and the surfaces of the animation.
Rigging is what allows you to move the character in a 3-D space. You can combine the proxies and the rigging together in a 3-D space for what amounts to true animation, whether it's computerized animation or hand-drawn.