How is the person selected? How far in advance is the selection process made? When a department makes an offer to have a speaker come, how often is the offer rejected? And what happens if the offer is rejected?
At my department, we often have talks by speakers who are already visiting anyway for example to serve on a PhD committee or as part of a collaboration with one of our researchers. This is usually a good opportunity to invite them to speak as they are already here anyway, their travel paid for by the PhD defence or by a research project of someone they are visiting.