Backup plans for speakers
You must secure a Plan B in advance in case you won’t be able to the one your speaker won’t be able to perform.
Making room for plan B
You can contractualy ask to your speaker to come with someone else able to make the presentation if he has a problem which prevent him to get on the stage or if he just faints. It could be wise to have a list of backup speakers too so you won’t have to cancel the event. They could be academic speakers which have the same knowledge as professional and usually cost less.
Using resources
Different sources of backup can be use. High-profile attendees of your event might be able to conduct a discussion or to give their own point of view about the subject.
Proactive check-ins
You must check with your speaker, a week ahead the event that there is no personal and professional problem for them to perform on the D-day.