At the moment, our three year old is doing lots of what is known as imaginative play, something I believe all kids do. It consists in him telling himself a story of his own invention, adopting one of its characters and acting it out with gusto. All the while totally absorbed, disbelief suspended, just as you are when you're watching a great film or play.
It struck me what a wonderful and peculiar thing this is. Extemporising playwright, avid actor and rapt spectator all in one.