If I interview a professor, should I pay him or her? As part of my Toastmasters club, I am preparing a speech where I have to research a topic I am not familiar with. One suggestion is that I could interview an expert (e.g. a professor) in a field related to the topic I choose. If I do interview a professor, should I compensate the professor for his or her time? What's the protocol for this? Does the answer change depending on whether I use the interview for commercial gains or not?
Unless you're giving a veeeery long speech or speaking about something highly technical to a group of experts in the field, there is no need to interview a professor. What you're most likely trying to do is give a layman's introduction to a fairly broad field. By contrast, researchers are highly specialised in very narrow fields. A short speech is unlikely to get to any details that you need to speak to an expert to learn about.

You mention that you're supposed to speak about a topic you're not familiar with. That means you don't have the background knowledge about the field that makes speaking to an expert useful. You would most likely get more useful information from Wikipedia, blogs (possibly by professors) or finding an introductory textbook in the library. Asking a professor to give you an introduction to an area you're unfamiliar with (outside their teaching duties) is kind of a waste of time for them. If you want to waste their time, then you should certainly offer to pay them, or at least offer to take them out to a lunch you pay for while they talk to you.

Like @cag51 metioned in the comments, you'll probably have more luck with a grad student. We're still new enough to the whole "expert" thing to be flattered that you would ask, tend to have a bit more time than professors, and never say no to free food.