Say your expert’s name is Kayla. Start by just saying “Kayla”. Pause about half a second until she makes full eye contact, until she acknowledges you. Then ask your question. By doing this, it focuses the conversation on you and Kayla. If Paul interupts you can politely say “hold on Paul, I want to hear what Kayla has to say”. It’s not impolite because you “set up the rules” for the question when you started.