Share everything you do for the preparation, such as documents or any research you perform prior to them. I would even share any pre-meetings that you schedule before the client meeting to help you prepare.

You would never want to share that you don't prepare or you don't think it is important to prepare. The interviewer obviously thinks it is important if they are asking this question. Even if you don't do all of the preparation, share all the parts that you do take on.

"I have a checklist that I follow in order to prepare for all my client meetings to make certain I don't miss anything. Of course, I may need to cater to that based on the type of client I am meeting with, but a majority of meetings follow similar agendas. I prepare an agenda, meeting materials, and even schedule a couple of pre-meetings with other departments to make certain I have the best foundation for the meeting."