Dwell on the details. Show your character in action. What are their likes and dislikes. I once read a story where the hero was afraid of spiders. This brought him to life. I often have characters who like a particular food or drink. Think of the millage you can get out of someone who craves sweets but not any sweet but a particular one. Think of how say a person who likes coffee. How about the hero who roasts and grinds the beans and blends the different beans. Makes for in interesting character. What about the heroine who goes crazy over silk. Think of how you can have her react to the cloth. It's details like this that turn your characters into living people.
But too much detail can ruin the picture. Remember who is describing another character. A hero will dwell on most likely part of her body. A friend on something else. The heroine will think of perhaps what's she wearing but not in detail. An enemy of adversary will use unflattering terms. All these make the character seem real.
Be specific - a word or two being specific will showthe character in truth. We've all read stories when the writer has written volumes about the meeting of the hero and the heroine. She will have the heroine describe every nuance of the hero's appearance. The reader usually yawns. or when the writer reaches the end of the monologue thinks why didn't she say that in the first place.
Think about the character's emotions when giving a description of them. Are they angry? How do thay act, speak, look. Do this with each emotional scene and suddenly you're working with real people, not the stereotypes they began when you first created them.