I'm thinking of applying for a PhD at my current university. I know the supervisor well and thought it would be nice to drop by his office before I apply to ask him a couple of questions about the PhD. Mainly to just show my face and let him know I'm keen. Any suggestions on what kind of questions I could ask him? I have a few generic ideas but would be nice to see if anyone else has had this experience and what worked for you?

Ask what they think (viability of the project, or your rough conceptions of potential research)

Ask if it is within their expertise or aligns with their current research agenda (you are gauging for interest)

Ask if they would be interested (as above), or have the time to supervise you (they may be taking leave soon, etc)

I would keep it super informal, like over a coffee if that is appropriate, just bouncing some ideas off them

I would also highly recommend Destination Dissertation, their chapter on PhD proposal formulation is the best out there. It will help you work through a lot of initial questions, and you will be able to approach with reasonably better idea of what you want to do

Ask yourself WHY you want to do this PhD, and think about where you want it to map (Industry vs academia). A bit of reverse engineering is always helpful. Basically think about you dream profession, work backwards, and ask yourself does a PhD map to that.

Thats all I can think about off the top of my head. But willing to come back here and interact. Also make sure above all else, you are motivated to do this PhD because you really don't want to do anything else. That fire has to stay lit for a very long time.

Sounds like great idea! I'd familiarise myself with the background of the project a little (not too much - that is what the first year is for lol!) and just be very interested - ask the questions that come into your head when you are there as well as just pre-preparing some. I think the main thing that will impress them about you as a potential student is your enthusiasm and interest in the area. Doing what you've suggested is a great way to demonstrate this.