What are the key questions you like to ask and why? How would the proper answers to your questions look like?

For more senior roles, I'd ask more about their decision-making skills around the team and PMM. For example:

• Tell me about a time when your product launch budget was cut by half. What did you prioritize and why?
• You have 2 great team members both looking to grow their careers and you can promote only one. What factors do you consider in this decision? 
• Tell me about one of the products that you worked on that was not successful. What did you learn from that experience? 

For more junior roles, I'd ask about their marketing experience. For example:

• Your fav marketing campaign and why?
• You launch a new feature and there Is low adoption by your customers. What data would you evaluate and how would you respond?
• What do you think about the messaging on the website. One thing you like and one thing you don't like?