What happens in a thesis defense when the non-committee members leave the room? I've been to many PhD proposals and defenses, but I've yet to give either myself. At every talk I've seen, after the speaker finishes his/her presentation and the non-committee members ask their questions, we're asked to leave the room. I assume the committee then asks the speaker questions. 

Are these questions any different from normal questions you would get at a talk or do they function as a sort of impromptu oral exam? Do students prepare for them in a particular way or it is just assumed you'll pick up on what you need from your studies and work on your thesis?
I don't think there is any universal answer to this question.  I just defended about 8 hours ago, in my case there was a pre-defense and a defense.  The pre-defense in our program tells the student what needs to be done to successfully finish the program, the actual public defense is largely ceremonial.  When I met in private with the committee they just shook my hand and that was the end of it.

I am aware of other defenses, where there is an oral exam after the non-committee members exit the room.  But, from what I have been told the questions in this exam are largely for clarification, and to determine what needs to be done to graduate.  

I'm sure some others can clarify, but usually your dissertation advisor has your back, and you have already done the work at that point.  So the defense is more of a chance for you to share your work, than a second order comprehensive exam.

It really depends on your country, institution, department, and program.