Living family members can be one of the best free genealogy resources at your disposal. While odds are a single person may not know all of the information you seek, you can usually put pieces from several people together to get a full picture. Even if someone doesn't know exact dates, they may know other facts that can help you narrow the gap.
Things to consider when interviewing family members
1) It was not unusual for children to be named after a favorite relative of the parents. My maternal grandfather is named for both of his grandfathers, Thomas for his maternal grandfather and Walter for his paternal one. When interviewing family members, ask if they were named after anyone.
2)If someone is unsure on a date, ask if they can remember anything that might help narrow the range. When I interviewed my grandfather in an attempt to get more information on his grandparents, I was able to determine that his paternal grandfather and maternal grandmother died before his birth and his maternal grandfather died after he was born but before he graduated. Since I knew that the two that died prior to his birth were alive when the 1930 census was taken, it narrowed the year of death for them to an eleven year range.
3) Most families have skeletons in the closet. Some relatives will talk about them and others won't. If you are interviewing someone and they suddenly clam up, don't push the issue. Change the subject to something more innocuous and note the question down to ask someone else about. If you push the issue, you risk the family member refusing to answer any other questions you may have, not to mention blocking access to other family members who may be able to give you answers.
4) Ask about religion. While some families stay with a specific religion, it's not unusual for others to change. Church records can be an invaluable resource when trying to trace genealogy. You don't want to waste your time digging through the records of the wrong church.
5) If you have information from your research, ask about it. It may trigger a memory. When I asked my mother about her grandparents' siblings, she could only remember a couple names. After doing some research in census records, I went back to her with a list of names. Once she heard a name, she remembered the individual and could tell me about them.
6) Come prepared with some questions, but don't be afraid to follow the interview where it goes. Having questions prepared will help prevent gaps in conversation. The flexibility to let the interviewee choose the direction of the interview may yield some interesting finds, which may include answers to questions you hadn't considered.
7) Be courteous. Ask before you show up with your notepad in hand. Keep the interview brief, no more than one to two hours. If you have limited time to spend with the relative, make sure you take frequent breaks. Thank the person for their time and any photographs or mementos they give you or allow to scan or copy.
8) Consider recording the interview if the person agrees. If you are jotting down information by hand, you may miss something or inadvertently get something mixed up. It also distracts the person being interviewed. Using recording equipment allows you to focus on what the person is saying and interact in the conversation. It may also help the person being interviewed feel more at ease.