I waited way too long to act on it and the reason was I just felt I didn't know enough to ask relevant questions. When the urgency came, I threw together something that I honestly feel a bit ashamed about and I couldn't execute fully because of time.
Anyway, some thoughts:
* Do it chronologically, start with their birth, their family, the childhood, the house, the toys, the games, school... then their adult life, their work...
* Something I was interested in was how life was like back in the day, food, comfort, customs. Anything that interests you and that they may have a opinion or an historical perspective about.
* You may use world events to help anchor questions but it didn't work well for me.
* A good question template is "what was the best childhood/parenthood/travel/work/X memory?"
* Last, I added the themes (most I didn't have time to ask about): education, religion, regrets, health (especially hereditary issues), war...
* When dealing with memory issues, help if you can but do not contradict too much because it may make them give up. Depending on their mental state, the point may be just to hear them talk and not to have a detailed account of events.
* Cut the phone. I'm very upset that too many of his last waking hours I spent with him he spent on bullshit phone calls (old people get many).
Edit 2: If I had had more time, I would have sorted through photos and start discussing from these.