Watch people, notice what they do with their facial expressions, their hands, and their body language. Notice that slight head shake back and forth when someone is totally disagreeing or disgusted with everything being said. Notice the squinted eyes, tight lips just before retaliation. Capture the folding of the arms and the nervous sniff the boss makes after he just put you in your place in front of peers.
We all know someone with a nervous twitch, a nose pinch with a thumb and a first knuckle. One eye opened wider than the other, a shoulder roll as if injured. Wiry hair, bull legged, feminine, stocky, old, youthful, a head that seems too small or too big for a body. Nervously pacing the room, strung out on coffee, eyes darting from one person to the next. These types of descriptions help the reader to visualize your character. I steal characters from my everyday life. Sometimes I mesh two or three people to create one more interesting character. Have fun painting the picture. If you have written a story and cannot describe the character, go back and give him or her more personality.