Sounds simple enough, but keeping your opinions to yourself and truly listening, can be difficult. It's best for the facilitator to work from an outline of questions. Using prepared questions will help you cover your objectives. Be prepared to be flexible, though, as the group may head off in a direction that is useful, but unplanned. If you've got some participants who don't seem to jump right in, you will have to encourage them to respond.
Personality often dictates the ability to respond spontaneously.
Some people have to be drawn out. You might ask them directly to respond. Of course, the reverse is true. You may have a dominate person in the room. By redirecting your questions to a particular person, you might convey that it's time for someone else to share.