If your writing a life story, the skill is capturing all the details. When you are in conversation, you can get the gist of a story and then when it comes to writing, you find that it looks thin.
Be patient with follow-up questions and be prepared to go slowly.
For example, if he talks about making an important decision, ask more questions to get to the heart of it:
1 what were the alternatives that you considered?
2 who did you talk to about the decision -- what advice did they give you?
3 did you make the choice with your head or with your heart?
4 what was the decisive factor in making the choice you did?
5 looking back, did you make the right decision? would you do it differently if you had your time again?
Capture dates, names, place names, as accurately as you can, even if it feels like you're being pedantic.
Buying a first car: what year? what model? colour? who did you buy it from? how much did you pay? how long did you have to save to get it? first long journey? was it reliable? what faults did it have? what opinion did friends, family have of it?