I see lots of posts about how to make a character interesting and not just a cardboard cut out. That’s a tricky question to answer because there’s many ways to do that.
Dialogue is one thing in a writer’s arsenal to accomplish the task. How does he or she speak to others and is the clean or potty mouthed? Is there lots of sarcasm? All of them display personality.
Actions are another option. How does she behave? Does she get mad easily? Never upset? Cold as Ice?
Narration can also help you.
Best thing is a combination of all the above. Characters are sums of their parts-just like we are as people, so keep that in mind when creating your characters.