Prepare around the person's actual experience. Look at their office, practice area, career path, and any public work so your questions have context.
Use a few strong prompts rather than a giant list. The consulting coffee-chat questions guide has examples that are easy to adapt.
Your goal is to learn how this person experiences the firm and let them understand your interests. A good conversation should not sound like you are trying to force a referral in minute nineteen.
Have a concise introduction, two or three thoughtful topics, and enough flexibility to follow wherever the partner gives an interesting answer.