When I wrote to an elderly aunt, I included a mini family tree to show how we were related then I asked a few questions and left plenty of space by each one. I said at the end that I would be happy if she wrote on my letter if that was easier for her and send it back to me with any comments she had. This she did and she sent me a few photos, including the first one I saw of my grandfather. Shortly after, I arranged to go and see her and only a couple of years later, she had died.
I discovered from further research that the info she gave me wasn't 100% accurate, but it all had an element of truth in and was enough to confirm that my research was correct. I sent her a box of chocs to say thankyou!
PS I also got a long complicated story about how she and her husband were diddled out of some money in a will from 40
years earlier - "It wasn't the money " she said "It was the dirty way they did it"