A good speech depends on several factors, for sure it’s not enough to invite/hire a good speaker. While she might have good public speaking skills, willingness to engage with the attendees or the [good] habit of practicing her presentation over and over again, some of the variables necessary for success are external to her.
It is the role of the conference organizer (program director, curator, etc) to coach the speakers so that they get to know and understand the audience even before they meet, helping to customize the message that she’s trying to push from the stage to the receptors. Do they speak the same language? Do they have enough knowledge on the subject she’s presenting on? (a typical mistake is to deliver basic content to a group of experts or -even worse- to present highly complicated concepts to an audience that doesn’t have the faintest idea of what the speaker is talking about). Physical variables like stage and conference room layout, lighting, audio/video support are also very important to a successful speech.
Bruno’s advice is:
“Speakers deserve to be helped. They are experts in their subject matter [...] but you, the conference organizer, are expert on how to tell a story on a stage and you know your audience better than anyone else. So what you can do is help [...] the speaker frame and configure their talk for that specific audience and stage”
“Don’t be afraid of suggesting new ideas for a talk, even on subjects you might not be familiar with. You’re familiar with the format”
“Make the subject and the format have sex until they really come out to something great”