Introduction to Character Design

How do you simplify characters so that they are easy to animate?
This is kinda a complicated question.  .... the main jist of character design comes from making the shape, outfit, pose and color of the character convey their attitude, or what you want the audience to THINK about them.  Alot of this comes from the overlying shapes that you use to construct characters.  You should be able to break up just about every character ever designed into a few basic shapes like spheres, triangles and squares.  In fact, if you simplify them (squint your eyes) some characters really are just a big square, big triangle or a big sphere.  The shapes that you use to construct your character with are going to help the audience understand what "kind" of character it is.  This may be obvious to them, or it may just be a subliminal idea they get by looking at your character.  Certain shapes have certain connotations to the brain.  A character that is mostly in the shape of a big sphere will be seen as either soft and cuddly, pleasing, dopey, or just plain fat depending on how you use it.  A large squarish character is going to immediately seen as strong, dependable, perhaps dumb or solid.  The more angular, triangular character will be seen as "active" or aggressive, sometimes evil.  You can use all this in their literal meanings, or use them to determine what YOU think they should mean.  The same works with color theory.  Certain colors have certain feelings attached to them for most people.  You as the creator though can decide to use them in a certain way.   Just do it consistantly throughout your peice.  These are just a few basic examples.  I'm not telling you to make steriotypical characters, I'm just saying that you should be aware of these preconceptions before you design characters.
Try to build your characters out of a few main basic shapes.  If your character is soft and delicate, you may want to make it out of soft shapes, and less angles.  If your character is the abrasive villain then design him so that he LOOKS angular and abrasive and jarring.  Perhaps your "abrasive" character has more points, more straights and diagonals than your more "soft" character.  Your character should LOOK the part, unless you are intentionally trying to trick the audience into thinking the character is something they are not.  Another thing to keep in mind is that often when you are designing characters to fit together in the same environment, they should all have a fairly similar style.  Remember though that just becuase they are in a similar style does NOT mean that you have to forego contrast.
Constrast is one of the most important things to consider when designing characters.  Often a good idea is to contrast a large bulky character with a small thin one.  Or a very young character with a very old one.  Noone wants to see five big burly handsome man characters whose only difference in design is their clothes and their face.  Make their WHOLE body shape completely different.  Give your audience something to intrigue them.  It is the same idea that comic teams use for standup comedy.  Two characters that contrast well against each other are more interesting than two characters who are exactly the same height, build and color.  It's not just comic teams that should contrast though.  Often the villain and the hero, the love interest and the hero, all  have radically different shapes and heights.    Beauty and the Beast would be a good example of this.  Look how Belle looks when she stands next to Gaston.  He's not only taller than her, but he's bright red in color as opposed to her subtle blue, more angular and (of course) radically more built.  Now watch when Belle stands next to the Beast.  Once again, her color contrasts with his dark cape and furr and of course once again her height and build contrasts with his.  In the ballroom scene, even though they change clothing and now Belle is gold and Beast blue, they still radically contrast in color.  I'm sure there are other films that would give you a better example of contrasting character shapes and colors, but I think you get the idea.
While on the subject of color, certain colors denote certain things about a character.  In general, red is exciting, daring, jaring, warm and sometimes even evil.  Yellow is cheerfull, happy, bright, eyecatching and usually a rather neutral color.  Blue is cool (sometimes COLD and DARK!), soft, and the sometimes the color of life (water).  A royal blue is usually masculine while a light blue can be a very feminine color.  Green is a lively color but also the color of money.  Pink is soft and feminine (sometimes overwhelmingly so).  Purple is associated with royalty in its pure form and usually is another feminine color in the pastel version.  Orange is rather bright and intelligent looking (if a color can be intelligent).  Greys are rather dark and neutral and tans and browns are earthy colors. Be aware of these connotations when you pic colors and hues of these colors.  (and we all know that most self-respecting villains prefer darker versions of these colors).  Just becuase these are the average color notations doesnt mean that you have to use them this way.  You can make your own color key, just be consistant in how you use colors throughout each project.  If warm colors represent your "good" side then keep it that way throughout the film and vice versa.  Sometimes a villain would where the opposite colors than you would expect, but in general the audience is only going to pickup on his color-meaning subconsciously... so it's usually best to make his dressing colors give the audience a hint to his true nature.  The same is true for the other characters of your film.  Just some ideas.
One other thing about color is that you should try and not use every color in the rainbow on each character... it becomes crouded, tacky and just overall confusing.  Using colors wisely and with a reason (whatever reason you choose) will make characters easier to read and more appealing to your audience. (and yes even villains should be appealing and at the same time evil.)
Back on the subject of simplifying characters though for animation.  Just some common sense points will often help you out here.  Do you really want to animate a character who wheres 50 necklaces, an elaborately embroidered and be-jeweled coat and completely drawn chain link armor?  Not likely!  Think about having to animate all of those separate jewels! Kinda takes the joy out of animating, doesn't it?  Redrawing monotonous little shapes exactly the same in each frame of animation is TERRIBLE!  When you are drawing concept art or just still drawings for Fun, then sure... give your character super elaborate armor and twelve daggers all completely encrusted with jewels, but when you are designing characters for animation sometimes less is more.  Remember those basic shapes I mentioned earlier.  This applies to clothing and accessories too.  Think about how much you can get across with the least amount of detail.  To some degree computer animation allows you to by-pass this simplification since you don't have to redraw it in each frame, the computer does it for you.  However, characters that have been condensed into their essence show up better on the screen anyway, so my suggestion is that no matter what medium, traditional or computer animation that you simplify your characters to what they really need to express themselves.  Don't add a bracelet just to add a bracelet.  Add a bracelet becuase it adds to the personality of the character.  A character like Esmerelda, from Hunchback of Notre Dame, just would not be complete without her bracelets and trinkets.  Just like with color, add accents and items for a reason.  They dont have to be my reasons, but they shouldn't just be arbitrarily added just becuase you happen to like characters that have lots of earrings and bracelets.  Something you might want to look into is comic book characters that have been adapted for animation.  See how the professional tv animation companies prepare the characters for animation.  Spawn might be a good example.  They simplified the style of the comic down to what ends up being a very effective statement for the tv show (on HBO). (Please note that I'm not suggesting you go watch Spawn. I don't particularly like the show, its just an example of characters prepared for tv.)  I'm sure there are other examples that you can find.  One of my other favorites is Batman: The Animated Series and Batman and Robin.  Both series have good use of color and simplification.  They use colors when they want to and for a reason, not just to add another color.  Check into it and see if you can disect the way they are using shapes and colors.  Find a show you like and see how they simplified it.