The characters were drawn with variants of facial expressions and body positions so that the animating would be easy later on the process. All the moving parts of the character were made separately so that they could be animated by moving one part at a time. That way the rest of the drawing won’t become distorted when moving hands, for example.
On the video below, you can see the steps of animating Roosa Ruuti. The mouth, the eyes, the brows and the hands all had their own “bones” that were moved to the desired positions. When the positions transitions smoothly to one another it turns into animation.