The 'fear' or the 'concern' you have raised is indeed a very valid one i.e., 'what if I don't know the answer!'.
Just after my conversion, I do remember the days when I would be witnessing to people of all (or No) faiths and they would very often ask me this question, "You claim Christianity to be unique and Christ is the only way! But question is, do you know enough about the other 'faiths' or 'religions' that you have rejected or if not, at-least you have implied through your claim for uniqueness of the Christian faith". Oh, I do admit now that my answer back then was not very convincing and I would feel very ashamed about it. I would feel as if I have let the Lord down! However, today when I look back, I can't deny the fact that it was those difficult questions or conversations that I have had, that has shaped (for the better) my understanding of my faith in Christ. Hence, very often I would say that it is my interaction or conversation with friends from others faiths that has drawn me more into the discipline of Christian apologetic. This has led me to read widely, listen hard and to talk to people often.
Coming to your actual question during the context of an open forum wherein I don't know the answer to a question and what do we do then....? Well, let me first admit that I have been in situations like that and during that time I have very often taken the following steps;
- First, my attempt would be to try and understand the question. To do so, I would come up with a few questions that I would throw at the questioner in order to better understand the question as well as the questioner. For instance, I would begin by appreciating the questioner for raising the question and then perhaps probe a little further with regard to the question itself. In doing so, one would gain more time to think before responding.
- Secondly, I do believe that there is nothing wrong in admitting that one does not have all the answers. In doing so, I believe that we are rendering ourselves vulnerable. Now, having said that I would like to clarify my position that while handling a difficult question, I always try to distinguish between two things in terms of my response. One is the question with regard to the most essential central claims of the Christian faith in line with the world-view questions such as "origin, meaning, morality and destiny". I always make sure that my response to any of these question is clear and tight without any doubts. There is no compromise in this regard.
- However, in my experience the "don't know answers to some questions" often fall in the second category (or the peripheral questions to any world-view) which I believe does not stand in the same level as the first one i.e., the essential ones. Again, this is not to say that these questions are not at all important but we need to probably respond by saying, "well, I am not sure about this or that but nevertheless let me get back to you on this...."
- Lastly, in my experience of conducting open forums on any (chosen) topic, the questions we are asked very often are predictable. Therefore, we just need to plan well in advance answers to some possible questions, perhaps writing down outlines of those answers on index cards. The more one does (i.e., speaking) the more confident one will become through exposure to some key (and important) questions.
Hope this is helpful.