I email them first with questions I have. I always give them my phone number in the email so that they can call or write depending on how they prefer to respond. A lot of times they will call me. Sometimes they respond with a good time for you to call them.
I guess I don't think of these people as being famous. I look at it like a client calling me. I would never want them to hesitate to ask me a question, so I don't hesitate to ask these authors about their work.