The audience are from all sorts of backgrounds, but tend to be well-educated. Most are interested in ideas more than facts, and are there to give their minds a healthy aerobic workout. We are not yet reaching the people who come to the pub to shoot pool..
Good subjects seem to be: current, possibly controversial, accessible. People don’t necessarily have to know anything about it. A good example: The Origin of the Universe. We don’t know anything about that, but just hearing the title, we have an armful of questions: How many universes are there? Is ours just one of a series? When’s it going to end, and how—with a bang, or a whimper? And the ultimate question: Are there signs of intelligent life in the universe?
To some extent your choice of subjects is dictated by who you can get, so become familiar with your regional science community. If you’re in a big centre, check journals like Science and Nature with an eye to your town, state, or province. Read the local newspaper, and special sections in national papers like the Guardian, Globe and Mail, or New York Times, both print and on-line versions. Look at the local college’s web site, read the faculty lists and their research interests. Do the same for the museum. Go on line and check the website for the Chamber of Commerce or other development board. I went to a college town in a small state a while ago, and was told “We’ll never be able to sustain a Café here, there is so little technology in this area.” So I showed them a list of 95 high-tech companies in their county that I had downloaded from their Chamber of Commerce site, each with a potential speaker in the shape of a CEO or chief science officer.
Subjects that are too controversial, and may end up engendering bad feelings if not outright fights. Most of these aren’t really scientific controversies, for example, Mel Gibson’s latest film, abortion rights, or euthanasia (but a scientific discussion of brain death might work). If there is science at the core of the controversy (GM food, nanotechnology, global warming) then these are good topics, even if your speaker takes only one side of the issue. You can always invite a second speaker for the other perspective.
Some subjects may be problematic, as there are people with strong but not necessarily scientific opinions out there. For example, there are people who see immunization as an invasion of their personal freedoms. There are those whose views on genetically-modified foods are passionate but not scientific. There are Creationists. Nevertheless, immunization, genetic engineering, and evolution are perfectly good topics, since most people who come to the Café seem to accept the scientific approach to evidence, even though they may have some reservations. And it is good for everyone to hear these different opinions.
Sadly, many of the experts one knows are expert in something so specialized or minute that, although they would leave an audience at the annual meeting of their professional society gasping in awe, they would paralyze a Café audience, no matter how good the beer. Is your subject “The role of transcription factors in the regulation of MAP kinase cascades?” If so, you will not be a hit at the Café. But could you lead a lively discussion on something like “Conversations between cells: How do we eavesdrop, and what are they saying?” If you can, your beer’s on us.