ask them why they became a teacher, or what inspired them to be a teacher. ask them what they wanted to be when they were your age, and what there favorite thing about teaching was. you could also ask them about the first time/school they taught at, if your doing a biography ask about where they grew up, and what there first job was before teaching.
You can ask the teacher about the best and worst experiences in his or her tea chin g career. Also ask about the most memorable events and most importantly about the achievements that they have accomplished.