This will depend a bit on who you are and what matters to you, because a man's death is more the survivor's affair than his own. You need to find out what you really want to know.

Of course there are the secrets but I know that the secrets I will take to the grave are not ones to protect me but to protect those around me, so you probably won't get too many real secrets out of them. What you may get is hidden gems that they simply forgot to tell you or secrets that used to matter but don't now.

In my case (but this is only me I realise) whether it was a grandparent, a parent or a wife that I buried, the things I wish I had asked are the information things, things that explain why things happened, why life went the way it went for them and how they became who they were because those things also shaped me. But I always did like to understand history.

I recommend just talking lots about their lives and the questions will come to you, along with other unexpected information that leads to more questions. If you are talking about them because you are interested in them they will reveal much more than if ask a few big questions that you want to know.

I would say how you are thinking that time can be short and you want to know all about them before you came into their life. A few good chats starting with their childhood will open much up.

You can also ask them "Is there anything that you think I will benefit from knowing about you? about how you see me? about how you think I can improve and grow as a man? Don't ask this one until they are already talking freely and give them time to reflect on their answers by coming back to the subject again another time.