Start on a human level. Create an index card for each character and list that character's motivations, desires, goals and problems. If you can draw on the lives of the people you know. Make notes about how your characters interact with each other.
Discard stereotypes. Using stereotypes about African Americans is problematic on a number of levels, not the least of which is that it makes a character into a caricature, robbing it of all humanity and authenticity. Your character can speak, dress or behave in a certain way, but the choices must be based on genuine people, and not some unsubstantiated idea you have about African-Americans. Do not base your choices on something you've seen in a film since you might be repeating a stereotype by another writer.
Research dialects. There are dozens of regional African American dialects, each with its own lexicon and pronunciations. When you write a character's biography, decide where he comes from or where he's lived for some time, and fit him to the proper dialect.
Think of people you know. Write from their perspectives to portray a living individual with human virtues and flaws, instead of a social demographic.