Choose the personalities who are most likely to play along.

Don't worry about whether someone fits whatever stereotype of a creative person. Go for the right attitude: minimum ego and open-mindedness are more important.

People who can be in the moment, with minimal ego, and who do not feel the need to pull rank (reinforce the hierarchy) so as to limit open discussion.

The good news is most people do enjoy and contribute to well-run brainstorming sessions.