- Don't shun stereotypes - While the overuse of stereotypes is definitely a no-no, but to utterly ignore every existing paradigm for character creation isn't such a great idea either. Despite what people say, they actually do like the familiar and dragging them by the hair into totally new territory probably isn't the best way to connect to your readers.
- Appearance matters - You have to give your reader some idea of what your character looks like. This doesn't mean giving the reader an info dump two pages long that ends up describing every single mole. Give them enough to form an image and then move on (and if you revisit physical appearance again be sure you are consistent).
- Dialogue rocks - Dialogue is where the reader has the chance to hear the character speak in the words that they have chosen. Unless the book is narrated by the character the reader does not get the chance any other way. That means the dialogue should be authentic to the character and it has to be distinguished from other characters.
- Everybody has a past - unless you sprung from the ground about a sentence before the beginning of the plot. How much of the character's past you choose to explain or explicitly detail is up to the individual writer and plot but every character has a past, has opinions and viewpoints and ways of doing things. Characters that seem to exist only for the sake of the current plot never really feel genuine.
- Relationships are necessary - You character is going to be interacting with others and it is important that you understand the relationship that they have with each of the other players. Is there a history? Is it a newly established connection? Are there other connections between the characters? If the relationships don't work then the characters won't feel right.
Thy Dialogue Does Sound Strange - Nathan Bransford
Writing Tips - Lyrical Press - Mary Murray
Reader Turnoffs - Elizabeth Spann Craig (some very good advice on other topics here as well)