Begin your development of animated characters by familiarizing yourself with what has already been done. Online video and animation websites are an invaluable resource for finding this information. You can find archives of everything from early 20th century Betty Boop footage to the latest cutting-edge computer animations. While watching the work of the masters, resist the urge to get lost in the plot and pay attention to the structure and craft that underlies the characters. As you become more familiar with animation techniques, you will be able to see more clearly how the animators create various effects, shapes and motions.
Keep a notebook of all your ideas. Don't trust yourself to remember them. Carry your notebook with you wherever you go and sketch out ideas as soon as they come to you. Over time, this notebook will become a valuable tool and a source of information and inspiration for the development of new characters. Keep colored pencils with your notebook so you can include detailed multicolored records of your ideas.
Schedule periods of time for yourself to let your imagination flow and throw out ideas with no restrictions at all. If you work with a partner, this technique can be even more effective. Sit down together with large drawing pads and talk and draw to see what comes out. The only rule in a brainstorming session is that there are no rules. As soon as an idea presents itself, whether it's a fat red man 20 stories tall or a colony of intelligent ants with bison heads, jot it down. Write the idea down in words and then sketch any images that emerge from it.
In the process of developing your characters for an animated series, choose a few from your notebook that you think have the potential to interact well with each other and develop them. The interaction between the characters is at least as important as the quality of the characters on their own. There has to be a creative but viable storyline between the different actors in a series. Draw out your characters in a storyboard, modify their appearances and actions at will and see what comes up. As you work on them over time, relationships will emerge that you can develop into plot lines.