Story people should portray someone who could be your grandmother, the old man next door, an old teacher or a neighbor, in other words, real people, people you would like to know or personalities that remind you of someone.
This goes for physical attributes, too. If the heroine thinks of herself as exquisite and beautiful, she’s probably not someone we’d like to be friends with. Besides, those are usually point of view issues. A person doesn’t look in the mirror and think of her own feet as tiny or that her lips are shaped like rosebuds. More likely, like Stephanie Plum, she looks in the mirror and thinks her hair looks like a Brillo pad. Let the other characters do the observations while you’re in their heads.
The most efficient way to write a walk-on character is to use stereotypes, because the reader already has an impression. This, of course, depends on the length of your story and the importance of the character.