A familiar character from a book, movie, TV show, play, etc. might represent the characteristics you attribute to that person. For example, if you dream about Harry Potter, whom you consider courageous, he might represent courage. This could mean you are admiring courage or wishing you had more of it, or you've experienced in somehow in your life recently, etc. A famous character can also represent fame or celebrity, or the actor themselves, or you may be dreaming about them because something reminded you of them lately.
For clues about what the person represents, consider what they mean or signify to you in real life. Also consider their characteristics or qualities that stand out in the dream, what they're doing and how, and your emotional reaction to their presence.