Your characters cannot exist in a void. Their ethnic background affects who they are, what they think and do, even how they talk and what they eat. It influences how they interact with family and friends.
For example, my family is a mix of all things European. We have no strong cultural ties to any one people or country. As a consequence, we have no traditional foods or traditions that we relied on growing up. Instead we were influenced by whatever family routines that my parents grew up following.
That is not true for many of us, however.
Consider your own heritage. Are there things that you do because of that heritage? Foods that you eat? Prayers that you say? How much time you spend with family and what types of activities your family shares?
Your characters have a heritage as well that most likely influences their behavior.
Your task is to create that heritage for at least one character.
Begin by doing some research. Find out what kinds of foods would be eaten in the country from which your character’s family came. What about traditions around holidays? What and how did they celebrate? What are attitudes about family? In some cultures family is a tight knit unit that comes together to celebrate or to mourn.
Make a list of important details. The more items you put on your list, the better.
Print up the list. Strike out those that you might find difficult to include in a story. Encircle those that are critical to the creation of a three-dimensional character.
Number the circled items from one to five, with one being the most important.
Tie each of those to a specific behavior that your character would exhibit.
Your next step is to write a short scene in which your character interacts with another character, one that is dissimilar in terms of ethnic background. Do not mention ethnicity in any way, but rather let a character’s background come out through the way your character speaks, thinks and interacts.
Don’t write in dialect, but think about word choice, sentence length and diction.
There should be substantial amounts of dialogue.
When you are finished with your scene, reread it to see if you can distinguish one character from the other. If you can, then you accomplished the task! If not, then rewrite, changing things enough that differences come through.
Have fun with this one.