The independent animation film maker who is doing everything himself will sometimes design a character during the storyboarding process. In such a case, it is equally okay to be a bit careless about details and accuracy, because the character design is still fluid.
By the time real animation begins, however, it is well to have a model sheet made up. I have a good example of that here.
I have a number of short scenes featuring these two characters, and I intend to animate all the scenes as a group. This is a good way to minimize a tendency to keep on designing as the work goes along; if I were to do one scene in the group now and another six months later, there would be a likelihood that I might have trouble getting the character to look the same.
Figure 1 shows a collage of storyboard images of the two characters I am calling Ben and Bev. They are male and female security personnel at the airport, in charge of moving people through the luggage X-ray process. Here, Ben's images are more consistent than those of Bev, whose hair style keeps changing through the sequence's storyboard.
In Figure 2, I have retraced all the images from Figure 1, plus many more from a second sheet, working to make consistent all the details and proportions as I drew. The result is a model sheet that will definitely help me to keep these characters in line for all of their scenes.