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. :-)