I've been reading Hawksmoor by Peter Ackroyd. This is one of Ackroyd's many London novels mixing past and present and is good fun, although I preferred The House of Doctor Dee. The title character, Inspector Hawksmoor, is in a couple of scenes before Ackroyd gives him a physical description. Because I'd already gotten to know the character, I'd already created a mental image of him. I pictured Inspector Hawksmoor as looking like Inspector Finch from V for Vendetta. The actor Stephen Rea brings the same sort of weary determination to the role of Inspector Finch as I saw in the character of Inspector Hawksmoor. Both hate their jobs, both are beaten down by the world, and both are dogged, intelligent police officers who will get their man (sort of).
When suddenly Ackroyd wants me to picture Hawksmoor as balding with glasses, my mind rebelled. I already "knew" what the Inspector was supposed to look like, and I didn't need some author to tell me! I always put faces on the characters I read about, usually using faces I've seen in the media, on the street, or even my friends. Some of my friends would be surprised at the roles they've played in fiction! And no, don't ask. I won't tell you. The point here is that if you want the reader to have your image of one of your characters, you need to establish it right from the start, and even then it doesn't always take hold. I've reread books and discovered that my mental image of a character had little to do with how an author described him or her.