"All good books have one thing in common - they are truer than if they had really happened, and after you have read one them you will feel that all that happened, happened to you and then it belongs to you forever."
Don't necessarily get a respons, but for sure can trigger the better ones :-). I think a nice explanation of what a book is doing on a location can trigger nice responses and maybe persuade a person to respond in the first place. Once I put a random book under a lion statue I stated the lion was guarding the book. I got a response it didn't come to a confrontation with the guard and the book was taken and safe.
I think trying to get a respons on as many levels as possible (Book, content, cover, location, theme (obvious for the non-bookcrosser finding it), QR codes) will enlarge the possibility of triggering a respons. Even then, some people just will not even look on the website. I can't blame them. At least I enjoyed releasing the book.
Our butcher had a display table out side his store all through December. I could not help myself and put a book on it " yummy cooking in december ". It has been there ever since. I guess people think its part of the display. :-)