Does your conference speaker have a pre-speech questionnaire, which is usually a document of two to four pages that he/she submits to you far in advance of the presentation? While you might look upon this document as busy work, it's actually a key tool in ensuring that the presentation comes off as planned.
The professional speaker has taken the time to craft a document that includes questions that will help him/her to better know your audience, your organization, what you're trying to achieve, and what message you want to have delivered. Most importantly, it helps the speaker to understand what participants should know and how you'd like them to feel by the end of the presentation.
If needed, the speaker should be prepared to cover this form with you on the fly, over the phone. Often, this will be the most convenient way for you to handle this task, and the speaker needs to be accommodating in this area.