We often use bits of people we know when writing a story. So the free for today is to take someone you know and see how many things you can learn about them. One of my granddaughters started this off by looking at her younger sister and using terms to describe her. Talkative, silly, loud. Those terms went on for a long list and gave one picture of the child but in this free writing exercise as an inspirational bit take one of the characters you're about to put into a story and start with the physical description. Height, weight, skin color, hair, eyes and anything else that's particularly physical about them.
Once the physical is down go to the mental. How do they think, what education have they had, what is their philosophy of life that you know. What kind of job do they have? Select all the terms you can think of . Just make a list and put it aside.
Go to the social part of their nature. How do they react to others this is where the terms my granddaughter chose for her sister come into play. List as many as you can see.
The fourth list will be their emotional nature. How do they react to situations. Think of them angry, sad, thoughtful and jot down terms.
What you have is a list of terms that describe your character. Now imagine them in the opening scene of the book and using terms see what you can do with the character, remembering to use the senses to bring them to life.