Persuasion is the act of persuading or convincing someone to believe something.
The role of a persuasive speaker is convincing the audience to accept his or her point of view.
for the last question :
1.The speaker has to convince the audience that they know what they're talking about. This includes knowing both sides of an argument and presenting each of them accurately.
2. It's important for the speaker to understand the audience to which they'll be speaking.