"I think for anybody reading the book they're going to get an idea in their heads of all those characters, and I think that once that gets fixed, it's quite hard to shake"

The quote recommends that the personalities in a book can end up being deeply deep-rooted in a reader's mind and be hard to neglect or change. Hugh Dancy, as a star and performer, likely understands the powerful effect that well-written personalities can have on an audience's creative imagination and also emotional link to a tale. As a writer, he may also be talking about the relevance of crafting compelling as well as unforgettable personalities in his own writing.