There is a tendency for the novice to simply design a character off the top of his head, without considering what physical aspects and personality traits seem to work well in combination together to communicate a clear visual statement to the audience. My view is that, before an artist can have any knowledgeable output, one must first have some informed input. Therefore, I strongly recommend sketching people in the world around you, either from life or, as is my preference, from studying various character types on video in order to build up a library from which to draw upon when designing a specific character. I prefer the latter way of working, as video provides a way of studying the subject in a completely controlled manner, allowing one to study the subject at one's leisure. It helps to see the subject in motion, which makes it easier to see the physical "design" of the face and body type in order to then exaggerate and abstract it. Additionally, seeing the subject in motion and displaying physical nuances through body language, expressions, vocal mannerisms, etc. makes the resulting sketches far more successful in capturing personality and inner life than one would likely achieve by working from a still photo image.
Anyway, here are some samples sketched from YouTube that hopefully will illustrate what I'm saying more clearly. It should be noted, however, that the goal of the sketches is not to come up with a perfect likeness of the subject, but rather to make an honest attempt at seeing a unique design in the face and body type and using that as a springboard for abstraction and caricature:
Here are the links to the YouTube videos I sketched these from:
Character #1 Character #2
Character #3 Character #4
Here are the links to these videos:
Character #1 Character #2
Character #3 Character #4
Ironically, the "tough guy" character I've sketched from the YouTube clip is an imagined personality suggested by the physical type, as the fellow in the video really comes across as a very friendly and gentle sort. But in animation and cartooning, perception can carry more weight than being literal to the subject. Remember, you're trying to put forward a visual impression that your audience will understand at a quick glance. When required, more subtlety can be developed though story and animated characterization as your film progresses.