Or, Avoiding the Longbottom Paradox
Readers who are invested in your story will begin to imagine the people and the places in it. This is what we want: for them to care about what you’ve created like it’s real. This means that, consciously or not, the reader is creating a picture in their minds. By adding detail too late on you can disrupt their absorption in your world by contradicting their image of it.
For example, if you want the reader to know that the character has an extraordinarily deep voice, or a strong accent, tell us that when they begin speaking. If you only share that after a significant portion of dialogue, the reader may feel as if they have been reading it wrong. Get your essential description in early to avoid reader upset.
It’s the same feeling you get when a film adaptation of a book comes out and the characters look nothing like how the source material describes them. Think of 6’5″ Jack Reacher being played by Tom Cruise. A brief Internet skim reveals that the opinionated masses believe that compared to the books, film Jane Eyre is too pretty; Katniss is too fair-skinned; Neville Longbottom looks too much like a young Clive Owen; and Tyrion Lannister is far too sexy for his (leather) shirt.
It’s fine for the reader to guess details that you don’t mention at all, it’s just when new information is introduced later that it can be annoying.
That said, you are allowed to surprise the reader on purpose, for example: ‘Betty swore internally at her alarm clock each morning, dreading another day of work. It was Monday and rain clattered into her windows. She dove further under the duvet; it was dreadfully cold. Well, no one can afford to heat draughty old buildings anymore. Eventually, the sound of her beloved dogs yapping inspired her to drag her weary legs out of bed. I suppose one must persevere, she thought, one is the Queen of England after all.’
What do you think? Has this ever happened to you when you’ve been reading?