Near the end of Chapter 4 ("Spontaneity") of Impro for Storytellers author Keith Johnstone explains:

The best trick I know for releasing the imagination is to persuade the students that their imaginations have nothing to do with them.

'The imagination is a huge animal with a will of its own,' I say. 'Be interested in it, but accept no responsibility. You're not its keeper. Where do ideas come from, anyway? Why should I say "I thought of it", or "I thought of an idea", as if my creativity was something more than the acceptance of gifts from an unknown source?'

Ultimately students have to accept that the imagination is the true self (as William Blake knew), but it's not easy to grasp this nettle.

^z - 2013-06-08