It’s not always easy to keep your characters’s physical and personality traits straight. This is particularly true for minor characters who show up several times, but aren’t ingrained in your mind like your protagonist should be.
One way to keep them straight is to keep a file or spreadsheet handy for reference if they’re not vivid enough in your mind without it. This also applies to certain scene locales. I remember reading a book one time where the color of the couch changed. Yes, weird, I know, but I’m the kind of reader who will notice such a thing. I don’t think I’m entirely alone with that, either. Alert readers will notice if your hero’s eyes are blue on one page and green on another.
If you’re writing a short story, this is usually not a problem. But if you’re writing a novel, especially a long one, this can become a problem. Consistency is important and this is one way to be sure you are without having to go back and find where you stated what the person looked like. Another help is making them so unique, as noted the other day, that you can remember.
Think about that for a moment, too. If you can’t remember, how will your readers? The one major difference there, of course, if that you may have taken months to write your book while a reader blows through it in a few days, making it easier for them to remember.
Whatever works for you if you don’t have a steel-trap memory, do it. This is another thing that can throw readers out of the story, a fairly major faux pas.