It's always easier to talk about things than to pull the breeze. The more difficult the professor is, the more concrete you want things to be. Could:
- Wait until the lesson has started and ask a legitimate question (as mentioned above Buffy).
- Read some of his research. Find out what they are interested in. Talk to your students and ask them what they do. Find something that you are really interested in. This is more risky since you might be really ignorant about it, but asking a smart question about something that the professor is really interested in (that is, not just the class) can give you great compliments.
I had a professor (world famous) who taught in one of my lessons. I thought it might be interesting as a consultant. My Dutch courage was to drink an entire coffee before going to talk to him about the problems he brought to the classroom and that I thought interesting. (Note that he was both a smoker and a coffee drinker so he appreciated the good conversations). We talked about topics in class (on Riemann Surfaces) and it was clear that, since he grew up, he was interested in most of them. At the same time he was known for not having taken many students and for being quick and surly.
In the end we did not mate, but seemed genuinely amused by our meeting (smiling and sitting to one side in the chair) and we talked for at least one hour of topics related to his class and areas for further research.