As a Facilitator, it's extremely important to give the presenter very CLEAR DIRECTIVES as to what is expected...and what is not desired.  In other words, if it's a five-minute presentation for the group, make sure the person knows they are to FOCUS ON THE MAIN TAKE-HOME MESSAGES, practise the timing beforehand, and give clear graphics if appropriate.  This is quite different from "Oh...talk about five minutes...whatever you want..."  Under the last set of directives, the person could easily ramble on for twenty or thirty minutes, wasting everyone's time.  Plus, the REASON for the directives should be given: such as "we have a tight schedule to get everyone in," or "after the five-minute take-home message presentation we'll have 10 minutes for people to ask you about specifics," and "your message will be much more effective if you have 1-3 PowerPoint graphics to back-up your claims."  Even experts in a field often are poor presenters if not given explicit enabling facilitation.