Seriously spend one hour alone with, unmoderated or whatever, no assistants, no managers, just you and them chatting about whatever...
1 rule: you are legally bound to never publicize anything said or heard, punishment being severe like possible jail time over 10 years (I KNOW fictitious but this whole thing is)
1 trick they know coming in you are a "fan" so they will probably be a bit reserved in their chat unless you are an excellent conversationalist

after a few posts I'll say who I had in mind and why...
but imagine getting DeNiro or someone alone, and what you could learn from them?