As with any GENealogy TREK, always start with what you know and work toward the unknown. Always take notes, and better yet - ask permission to also record the conversation either audio or video for future reference. I find it helpful to have family group sheets and a pedigree handy for reference, my memory is not always what I would like.
Pictures can also be a great way to jog a memory.
Sometimes a bit of information will have a whole new meaning when you review it at a later date. Still others will tire out quickly and perhaps need a second visit to cover all the information completely. They might very well look them over and remember an additional tidbit of information down the road to share with you. The birth info of a sibling might mention a clue to birth order, which is a clue for birth years. The birth info of children could hold a clue for the age of the parents.