If you want to bring something on stage with you – a new product that you are discussing, or your latest book – always ask the organiser first whether it will be acceptable. There won’t usually be a problem, but it’s common courtesy to get permission to use some of your speaking time (which you may be being paid for) to advertise your items.
However, there is another issue to consider, especially if you are demonstrating something technical. For some years, I had a regular slot as a technology expert on a weekly TV show. Much of the show was devoted to new technology, which I would explain to the presenter. We often used to encounter what Dennis Norden used to call the “Oops” effect (Objects Only Perform Sometimes). If your demonstration goes horribly wrong, as mine did from time to time, the only thing to do is to make a joke of it and explain what it should have done.
One way to minimise the risk of failure is to rehearse and test, but that will not eliminate the risk completely. Some objects are fine in rehearsal, but stubbornly refuse to work in a live performance. As with all things, you need a fall-back position (yes, you can make up your own punchline about falling backwards there). Imagine how you would cope if nothing worked as expected. One day, that preparation will save you a huge amount of embarrassment.
In some cases, devices that fail to work can make a career. The great comedian and magician Tommy Cooper found that audiences laughed more when his tricks failed than when they worked, so he switched to getting things wrong on purpose. In general though, my advice would be to avoid using props.
– Alan Stevens, FPSA.