Can a non-committee-member ask questions while watching a friend's PhD defense? I'm going to a PhD defense tomorrow and his topic and methods are sort of close to what I have in mind for pursuing myself, and I'd like to be an active participant during the talk; what is the typical etiquette to follow?  Do only the committee members ask questions during the defense?
This depends on your country, university, and department. In some places, questions by non-faculty are rare. In other places, the discussion can get out of hand.

As it sounds like you are only starting your research and that this is your first PhD defense, you may want to consider sitting quietly and watching and learning what happens. After the defense, you can contact the student, and any committee members, and strike up a conversation.