Publishers equip exhibition stand with terminals that allow to interact with (star) authors even if these are presently absent.
What is the core principle of your idea?
Visitors can virtually interact with authors' avatars using the same principles as applied to Customer Relationship Management contexts. Cf., e.g., the legendary Anna of IKEA.
Can your idea be applied to different formats (in a bookstore, at a trade fair, at a conference)?
Fits best to fairs.
Would it be possible to turn your idea into a reality, or at least create a prototype of it, at short notice, i.e. by the Frankfurt Book Fair in October 2014?
Possible. Just take existing software plus create Avatars of the most relevant authors.
What do you estimate the costs for the prototype or the final implementation of your idea to be?
Software acquisition (freeware?) and Avatar design.