Keith Johnstone is a British and Canadian pioneer of improvisational theatre, best known for inventing the Impro System, part of which are the Theatresports. He is also an educator, playwright, actor and theatre director.
There are people who prefer to say 'Yes', and there are people who prefer to say 'No'. Those who say 'Yes' are rewarded by the adventures they have, and those who say 'No' are rewarded by the safety they attain. There are far more 'No' sayers around than 'Yes' sayers, but you can train one type to behave like the other.