Curious, excited, safe and playful are some of the critical moods you want participants to have. It will allow them to create freely and comfortable. The question is; can you change the way people feel?
Creativity is something you can train and develop. Some people might find it easier because of their education or previous experience, but we can all be creative in one way or another. It's just a matter of practice.
Making sure everyone in the team participates in the creative process is a hard task. We are not only dealing with different personalities, but there are also many factors that could make some people quiet and others too loud.