Why Law Interview Question - ADVICE!!!

Why not?

"Honestly I was first drawn to being a lawyer because I saw it as a lucrative career that pays well. After doing more research/gaining more experience/speaking with lawyers/working in <job> I became particularly drawn to <area of law> because <reason>, and that's when I started viewing it as a job that would actually interest me day to day, and something that I could excel at."

Obviously there are many different variations to that, but hopefully it demonstrates that you don't need to come up with some sort of magical fictitious answer to that question. Contrary to popular belief, that question isn't there because there is a right answer or to test your dedication. It's there just to find out about you and what motivates you. If that was initially or still is money you certainly will want to go beyond that, but there's no reason not to be honest about it.
Sure you do. He's a senior practitioner doing a job that you want to do. He not only knows so much more about the job than you, but he does it in practice every day. Ask him about those practicalities. Ask him how he manages a particular difficult situation that might crop up. Ask him what he thinks the most satisfying part of the job is. Ask him what the worst parts of the job are. Ask him a thousand and one other things that will help you to gain an insight into your intended career.

Now actually you don't need to ask anything in a formal interview situation. Contrary to popular belief, it's not something that you gain marks for. I am assuming here that this 'informal' interview really does come down to something of a chat between the two of you. In which case you can quite easily ask questions such as the above by just slotting them in to the conversation, if appropriate. But really, don't feel that you need questions prepared in advance. If it is on the more informal side of things, just go with the flow and ask something appropriate to the conversation. Or if you genuinely don't have any questions to ask, that's fine too.