Give the reader free rein to visualise and they’ll do a much better job of describing the character than you can because in their mind’s eye, they’ll actually see the character.

This is so true. An example comes to mind...Margaret Mitchell's Gone With The Wind. When I read the book I could visualize the characters with a certain clarity. When I saw the movie, I was disappointed in a few of the characters because they weren't as I visualized in my mind. Actors fault or casting? Perhaps a bit of both...