Mock objects, it seems, have been in use in the film industry for decades. Actors who have done any considerable amount of blue or green screen acting - and especially actors who have performed in scenes with virtual, digital characters, will be familiar with the idea.

Sarah Bolger, who appeared in The Spiderwick Chronicles, describes in an interview the experience of "acting with tennis balls" that are there to allow the real actor to do a more realistic performance, in...