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.
Yes, you should expect to pay him/her (the real question is how much)
As with any expert whose professional services you are engaging, you should expect to pay him/her. The real questions are:
- how much; and
- whom you pay (e.g.: the professor; his/her university; an agency managing his/her speaking engagements; &c.).
If you have a limited budget and know that you cannot possibly exceed it, you should say how much you can offer at the time of writing, and hope that the professor of whom you enquire will agree to it (but ensure that it really is the maximum you can offer -- many professors have neither the time nor inclination to haggle, and will simply ignore your enquiry if the amount you offer is too little).
If you do not have a particular limit in mind, you can ask the professor for a quotation -- the amounts can vary considerably according to the professor and according to the context, so it is worth giving a brief description of the event you have in mind and your own position. Unless you are manifestly wealthy, giving such context will probably work in your favour, but do not lie (e.g.: do not say "this is a charity event" unless there really is a registered charity involved).
Once the professor expresses interest in the engagement, you should ask how/where to pay him/her. He/she may undertake external speaking engagements:
- as a freelancer (in which case you pay him/her directly);
- through his/her university (in which case you pay the university); or
- through an agency (I know several academics in literature and in music who have agents).
One final point: a professor may be subject to restrictions on the freelance/external engagements he/she can accept, especially if he/she is not a citizen of the country in which he/she works. If that is the case, he/she may agree to work pro bono, in which case you should offer to make a donation to a cause of his/her choosing.
It's understandable that you don't have time to googling around, and probably asking someone who knows the topic is much more efficient. Plus, if the format of the speak you are delivering requires you to have an interview with an expert, then probably you should prioritize searching for someone who agree to interview rather than studying the problem yourself. In that case, when you google the topic, try adding some keywords about the site that experts most likely frequent. Probably Reddit. Asking for help there, and probably you can arrange an interview with low cost.